The wiki should help but once you get it figured out, its easy to remember.
Build a bed, armor stand, or weapon rack.
Assign it as a barracks, then to the squad you want to train. (hit ZTIQ with the bed/stand & correct squad highlighted)
Hit m for the military screen, then S for schedules. Hit E to edit the orders, then change the minimum training amount to be equal or less than the amount of troops in the squad.
Once you had set the amount, you can hit C to copy, then hit P and paste it for every month of the year you want them training. You can hit cancel to insert months where they do nothing (ie breaks).
You can decide how you space their schedule, some people train for 2, then 1 off. Others train non stop, yet others give them equal breaks to time spent training.
IF you have archers/marksmen, you need to do 1 more step. Build some archery targets, set the direction for firing and make sure there are no obstructions between the targets and where the dwarves will be standing to shoot at them. Select each target, highlight the correct squad, and hit T to make that squad train at the target.
And that's it.
If successfull you should notice your troops heading immediately into the barracks, or atleast some of them. Usually they wont all train 100% of the time , they will take breaks, eat, sleep etc.