If you have N marksdwarfs in your squad, set up at least N archery targets.
Make each one into a separate archery range (they can overlap), and assign it to your marksdwarf squad.
Do NOT assign your archer squad ANY other barracks! Otherwise they will stand around watching dodging demonstrations, instead of learning how to put bolts in goblins from across a room.
Make sure your marksdwarf squad is active (m-a and set them to "Active/Training") and that it has the order "Train (10 minimum)".
Make sure your marksdwarf squad has been assigned crossbows. The default Archer costume works, although I use the following setup:
Leather Helm
Hood
Leather Armor
Leather Leggings
Cloak
Leather Gloves
Leather High Boots
Copper Shield
Copper Buckler
Crossbow
Again, if you have N dwarves make sure you have at least N of all these, along with quivers and waterskins.
For bolts, the default "Bolts[250] CT" works. Make sure you have at least that many bolts in your fortress, and preferably many more; I turn on the Weaponsmith labor for every immigrant without a moodable skill, and have them turn out a stack of copper bolts before turning off the labor. (That way if they get a mood, they will hopefully turn out an artifact weapon instead of stupid bone crafts. I have a magma forge filled with Craft Copper Bolts labors on repeat, and the Profile set for max skill to be "novice".)
You should get dwarves plunking away at the targets in short order.
Oh, I noticed you dug channels -- that screws things up pretty badly in my experience. They need to be able to path to the archery target or they won't shoot. I haven't played around with it in the newest version, maybe something like this would work:
#_.
#X_
#_.
#X_
#_.
#X_
#_.
Where # is a wall, X is an archery target, _ is a channeled space, and . is normal floor. Obviously these would be set to shoot right to left. That way dwarves can path diagonally to the target, but if they hit it (or miss to either side) the bolt will fall into the channel. Then set a ammo stockpile at the bottom of each channel.