What do you think of the plot that goes the bad guys announce they’ll come back to fight soon but the majority of the good guys have no clue how to fight and it’s up to a couple of people to train everyone asap?
I’m not wild about villains who announce their presence, and then wander off and give people time to get ready. I’m fully aware there are legitimate, character, and story reasons a villain might do this, it’s just something I just have a hard time buying that structure. The reasoning being, if your villain announces their intentions, someone will try to stop them. So, either they should keep their mouth shut until their ready to act, or they should act to suppress any resistance before they can finally enact their grand plan.
Should this matter to you? Probably not. This is just my personal taste. There’s certainly room for Saturday Morning Cartoon villainy that requires someone to announce their intentions. There’s even real world examples of this. Monty Python taught us that, “Nobody expects the Spanish Inquisition.” However, as it turns out, that was a lie, the Spanish Inquisition would file notice a month in advance, to give the accused the opportunity to secure testimony and other exculpatory evidence, (or put their affairs in order.)
So, it’s fine, just not to my taste. That doesn’t reflect on you, and shouldn’t impact your decision to write it.
The good news is, if you have a few characters with similar training backgrounds and a willingness to work together, you have everything you need to set up a combat training class. What, exactly, this will look like depends on the technology involved, and the combat doctrine the characters are following. They’ll need improvised training weapons, and (somewhat obviously) live weapons. (From a logistical standpoint, if your characters are using firearms, they’ll need at least roughly one thousand rounds per weapon to train the recruits, and then equip them. This is a factor that a lot of people overlook when trying to equip untrained militias.)
For melee weapons, you can begin walking the recruits through basic techniques, then moving to group drills. For some techniques, you’ll need to pair trainees against one another. In these cases having assistants who’ve already undergone training can work wonders for making sure that everyone is doing what they’re supposed to. For melee combat, the purpose is to drill the movements until they become rote. This means if someone is training incorrectly, they’ll be committing those errors to muscle memory. Assistants can be invaluable for finding and assisting recruits before these mistakes become ingrained. At this stage, the use of training weapons is preferable.
If you’re dealing with ranged weapons, then you’re going to need to commit time to training them on those weapons, in order to be able to operate them under the stress of combat. To a certain degree, some of this is the same. You’re getting them to commit acts like aiming, firing, and reloading to muscle memory. That said, they also need to learn how to fire accurately.
Beyond basic combat training, you’ll need to instruct them in basic battlefield tactics. This includes things like how to move through an area safely without exposing themselves to enemy attack. This will look radically different depending on the technology in use.
Your militia will need a coherent chain of command. This is really important when things start going sideways. The priority will probably be a simple structure where the most experienced combatants are spread out and can direct the recruits.
Parallel to this, the experienced combatants need to identify useful skills in the local population. This includes things like medical training, hunters, engineers, and someone can manufacture weapons and armor. Skills that can be useful. If a specific role isn’t available, the next best thing may have to suffice. For example, if you don’t have access to a doctor or nurse, a veterinarian can do the job in an emergency.
Specialists are useful for a number of specific functions. Some are self-explanatory (you’ll need medics to help treat the wounded), you’ll need builders to help fortify their location (aided by whatever materials are nearby, which may also involve miners or lumberjacks), you’ll need hunters as skirmishers, for reconnaissance, and possibly as trappers. Just because the villain said they’d come back doesn’t mean you should hold them to their word, stay vigilant and prepare. A smith can be useful for aiding in the fortifications, or assisting in arming the militia.
While having a well trained force is important for winning a battle, taking control of the battlefield, restricting how, and where, your opponent can attack, and using every resource at your disposal to undermine them is vital to victory. How your characters do that will depend on their ability to tilt the odds in their favor.
Your villain said he’d come back. That doesn’t mean your characters should just sit around waiting for the inevitable. They have time to prepare, dig in, and make sure that by the time the villain arrives he never had a chance.