A word of caution, this is based on my experience. It works for me; but, for all I know none of it's true. Also I've just barely started into the newest version there may be changes. And the wiki probably says it better.
A few guidelines I've found from my frustrations of dealing with the military.
1) Have the squad size of 3 for new squads. Once they're good at what they do you might want to consolidate them into mondo 10 man squads. Or maybe not.
2) Have each squad set to Train for 8 months & No Scheduled Order for 4 months. Break up that 4 months off time across the year - not all at once.
3) When set to Train make sure they are set to Train, 2 minimum & don't forget to copy that order for the other 7 months.
4) Have a barracks & set each squad to train there.
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5) In addition to all this, for each xbow squad, create 3 archery targets. DEFINE EACH TARGET AS A SEPARATE ARCHERY RANGE. ( This was the biggest "Ah Ha!" moment for me. Just defining one that overlaps the others DOES NOT WORK. Training really took off after I got this. ) Assign each xbow squad to 3 of them.
6) Assign ammunition for each xbow squad. I use "200" bolts per squad - that's 100 wood bolts set as Train, & 100 bolts of anything else that is not wood for Combat. You could go all wood all the time for everything.
7) Set up a massive bolt creation industry. You are going to be strip clearing the forest regularly for wood for bolts for training. You want at least 1000 wood bolts in your stockpiles, per squad, per season. Check every season at a minimum, you'll probably need more than this.
This should give you a functional military in a year, & some very dangerously effective ( but not completely legendary! ) squads in under 5 years. Any extra real combat you can give them, provided it's done in carefully controlled & totally unfair circumstances, will greatly accelerate this process.
Keep in mind this is just a guideline - someone else can probably pick this apart in much greater detail.
( Extra edit - I'm not even going to try to cover uniforms .. ask around. One last suggestion, have each squad set to sleep in their rooms, & don't let them carry any food or drinks. )