How do I even start to train marksdwarves? I haven't got a clue how to get them
I'm a newbie, but this is what I did:
1. Make 1 crossbow and quiver for each dwarf you want to be a marksdwarf, and a bunch of bolts (whatever material you want)
2. Create a new squad, and when the uniform option pops up, select "archer". Assuming you haven't got bows laying around, they'll pickup the crossbows and quivers + bolts
3. Then create an archery target and 'q' over it to make an archery range. While in the archery range 'q' menu select your squad and press T to train.
Another thing you can do is enable the hunting profession and then they can train while hunting too.