Please read Supplemental A first. Trust me, it makes more sense that way.
In the post Writing 101: 2-A we went over creating protagonists and antagonists. In Supplemental A we covered the 12 steps of the protagonists's journey. These are the 12 steps of the antagonist..Remember your antagonist will have gone through some of these steps before your story start. They should have completed 7 of them by the time the protagonist starts so that the protagonist's step 2 (Call to Adventure) lines up with your antagonist's step 8 (First Capricious Act of Evil) or 9 (Reward {Seizing the Gauntlet}). If the gauntlet your antagonist gets is the thing that calls the protagonist to adventure...beautiful. Instant-plot!

These 12 steps your antagonist must take are:

Act One (Separation)
1. Ordinary World
2. Imposing Disaster
3. Mental Breakdown

Act Two (Transformation)
4. Meeting the Dark Guide
5. Committing to Evil
6. Struggling With the Conscience
7. Killing the Conscience
8. First Capricious Act of Evil
9. Reward (Seizing the Gauntlet)

Act Three (Return)
10. The March Home
11. Battle With the Hero (Resulting In Grave Injuries)
12. Rebirth With the Prize

Again, let's not apply them to traditional villains of hero epics, but to antagonists of regular novels. Most of your writing will be romance, science fiction, fantasy, horror, thriller, or mystery stories. Treat this is a glossary of the above terms translating them into what they mean for these average category fiction styles. Let steps 1-7 be written into your character bio. They may be referenced in the story and they're important for you the writer to know, so be sure to plan them out for your main antagonist.
Separation: This is the initial event that took an ordinary creature or person and made them evil. This is where you must decide if your antagonist is sympathetic or unsympathetic. If sympathetic, they should go through intensely traumatic events and struggle with good vs. evil. If unsympathetic they should have overreacted to very minor events and setbacks.

Ordinary World: Decide if your antagonist was born with great potential for evil or crafted that way by life. Here is where you note if they are a hellspawn demon or psychopathic killer from birth, or if time and circumstance shaped them into the crooked senator, the blackmailer, the jewel thief, or the land-grabber.

Imposing Disaster: The first hint of their evil should have come in preparing for disaster. The actual disaster will come in the next step, but they should have had foreboding. Their home world was threatened with destruction, their step-father started giving them odd looks, their beloved was diagnosed with a terminal disease and doctors refused to aid, or they were destitute and wanted to get rich quick. Here is where you show how your antagonist reacted in a way that a protagonist wouldn't.Here is where they considered stretching their morals, or came up with a plan B no good guy ever would.  

Mental Breakdown: The inevitable happens: their world or plane is destroyed, their step-father rapes them, their beloved died horribly, or the divorce papers or eviction notice come. After, rather than have a minor breakdown and begin the struggle back to normalcy via healing and seeking justice as a protagonist would, the antagonist seeks revenge and power after going slightly mad. They seek to inflict pain and destruction to feed their fractured ego. In order to get to this point they need a total mental breakdown and a parting with reality that sends them floating into enraged, impotent chaos. This is when they drive people away and begin to completely divorce themselves with seeking normal, healthy intimacy with others.

Transformation: This is where your antagonist goes from little-boy-lost and becomes the Big Bad. This is where they learn what they need to achieve their goal of power and inflicting pain, find a guide.or inspiration/group, and concoct their plan that the protagonist must foil.

Meeting the Dark Guide: This is that moment when the villain is babbling madly then suddenly stills and says "that's it!" or is approached by a true villain who taunts them and challenges them to become their minion.It's where the antagonist gets ahold of a mentor or an inspirational figure. It's where they read Mein Kampf and join with Neo Nazis, meet the dark wizard or elder demon/monster, or find their more experienced partner-in-crime. They have no specific plan, they are emerging from impotent chaos, and this is the person or idea (an idea must always lead to joining a group) that will give them focus and set them on a path to learning the dark arts.

Committing to Evil: No matter how evil your antagonist is, they have a conscience. This is the last big flare of it, and the decision to squelch it. Often antagonists have a weak mind compared to protagonists so it's typical the impetus for evil comes from another. It's when they wince at drinking blood but the cult members call them chicken. It's where they consider maybe not taking over the world, but their overlord or mentor demands they do it or face death. It' where they rethink their blackmail but their partner threatens them. It's the coward's choice, and their twisted ego won't let them reconsider once they choose evil, as they now begin to feel any chance for a normal life is lost.

Struggling With the Conscience: The antagonist has Committed to Evil but this is not done in one fell swoop. The dark guide will give them a series of tests, each one harder than the last, to complete. Often these tests bring them in contact with "good" outsiders who try to talk them out of the evil path, and "bad" insiders of their new world who taunt and bully them deeper. Often they will try to convince the "good" outsiders they are evil, but inside they are convincing themselves still. This is the step where they cut themselves off slowly from the outside world and dive deeper and deeper into the underworld until only on "good" outsider remains trying to pry them from their dark path.

Killing the Conscience: This is most often an internal decision to commit to the next step. This is where they agree to do it, and two things usually happen. The last "good" outsider typically makes one final appeal only to be turned away, and a physical transformation occurs. The demon or monster is disfigured or comes into full power. A scar is given, a hand is chopped off, some physical disfigurement occurs that the antagonist sees as fate reaffirming their new status. The blackmailer, crooked cop, or evil senator makes a final pledge that is recorded, or are themselves blackmailed until they make peace with the next step to come. At this step they now have their pan for evil, but there is one last act to perform before they can strike out on their own and leave their dark guide.  

First Capricious Act of Evil: One final act is all that separates them from their imagined self as the Big Bad. Up to this point they were a minor demon, an apprentice sorcerer, a minor enforcer of the mob, an initiate of the cult, or a somewhat reluctant partner-in-crime but this is the act that will bind them to the dark guide and  cement their path forever. It's most often murder. In the last step it could have been killing animals, robbing banks, raping, stealing, blackmailing, or terrorizing people, but this is the final act, murder. It should ring of betrayal: If an unsympathetic antagonist they should kill the "good" outsider that stuck around the longest or someone pure and innocent like a child.If they are sympathetic, they should kill their dark guide mentor, partner in crime, or fellow initiate, making for some twisted kind of karma. In crossing the river to evil, this is the ferryman's price, the unforgivable sin that will make all "good" outsiders turn from them at last. This is the act they do that sets their fate on a collision course with the protagonist. 

Reward (Seizing the Gauntlet): In the last step their humanity was sacrificed. Now your antagonist needs a reward to spur them on and encourage them towards their ultimate plan. This is most often a token that will allow them to.complete their new  plan. This is the dragon stealing the princess, the James Bond villain seizing the  diamonds or government secrets, it's the mobster getting made, the jewel thief scoring their big heist, the crooked cop or senator getting elected or paid, or the blackmailer winning the big score. It's any kind of material reward that the protagonist cannot escape noticing and be called upon to retrieve.

Return: Our antagonist started as a twisted innocent betrayed in life, and now has trained, grown, transmogrified, and has something the protagonist needs. Now they attempt to return to the ordinary world and enact their plan, claiming their place of power they so desire, or rejoin with a partner in crime they trust.

The March Home:This is the antagonist coming home to roost, or trying to. It's taking over the evil step-dad's corporation or kingdom, it's the monster settling into their new home or claiming the old, it's trying to rejoin the partner-in-crime only to discover betrayal. It won't be easy. All those friends the protagonist is making during their step #6 (Tests, Allies, Enemies)? They are thorns in the antagonist's side that they easily evade/vanquish, but still delay things.This is also where the new inflated ego of the antagonist is a downfall; the march to the castle is waylaid by conquering every town they pass to feed their hunger for death and destruction, or they overreach their agreement and desire to become the mafia don, the president, or betray their partner in crime for an even bigger score. It's allies falling through and not holding up their ends of deals. It always leads to the loss of a crucial piece of evidence the protagonist will get in their step #9 (Reward (Seizing the Sword)).

Battle With the Hero (Resulting In Grave Injuries): This is the climax, the final battle, the final confrontation. At first your antagonist seems to triumph but that little fucking shit protagonist comes from behind and wins. Protip: never fully kill your antagonist, always give them a way back to life, which comes in the last step.

Rebirth With the Prize: Life sucks. Your antagonist tried and lost. But they get one last chance! If they go to jail, they have the opportunity to escape. If they get sent back to their dimension they may have a way back. If they die they must either have some way to reincarnate eariy, or if sympathetic they must achieve guilt and this go on to a rewarding afterlife. Perhaps they lose the battle, but screw the protagonist out of any expected reward and escape. Hey, even Darth Vader came back at a the evil teddy bear Ewok jamboree.

It's a good idea to give your antagonist as many steps as your protagonist. Be it 3, 4, 6, 9, or 12 they should be the same. However as we've seen, they will overlap. Here's a handy guide as to how, using 12 steps for each (protagonist steps are in yellow, antagonist in red):

Antagonist's Act One (Separation)
1. Ordinary World
2. Imposing Disaster
3. Mental Breakdown

Antagonists Act Two (Transformation)
4. Meeting the Dark Guide
5. Committing to Evil
6. Struggling With the Conscience
7. Killing the Conscience

Protagonists Act One / Antagonists Act Two Continued

1. Ordinary World / 8. First Capricious Act of Evil  
2. Call to Adventure / 9. Reward (Seizing the Gauntlet)
3. Refusal Of the Call                       "

Protagonist's Act Two / Antagonist's Act Three (Return)
4. Meeting the Mentor / 10. The March Home
6. Tests, Allies, Enemies                   "
7. Approach To the Inmost Cave         "
8. Ordeal                                          "
9. Reward (Seizing the Sword)           "
10. The Road Back                           "
11. Resurrection (Final Battle & Cheating Death)11. Battle With the Hero (Resulting In Grave Injuries)
12. Return with the Elixir  / 12. Rebirth With the Prize

There you go. Now you van visualize just how long that march home will be for the antagonist. This is why in fiction often the antagonist has had more mileage than the protagonist, is days ahead, or years older. Perhaps their march home meant amassing a fortune, building an evil fortress, stashing the jewels, or consolidating their unholy powers. Whatever it was, it takes a long time. Play with it! Perhaps their step #9 (Reward (Seizing the Gauntlet)) occurred in the protagonists childhood and they made their #10 (The March Home) and the protagonists #2 (Call to Adventure) comes when an od protagonist comes to find the new one grown.

Remember some general rules: in a mystery or thriller typically these 12 steps take days, but on occasion can take months or years. In romances and science fiction they usually take months. In horror and fantasy they typically take years. It is usually faster for the protagonist; typically it takes 50% of the time to complete 12 steps for the protagonist as it does the antagonist.

No matter what make it your own, just remember your antagonist should always be on step #7 or #8 when your protagonist is on #1.