If you want to use archery targets, the way to do this was answered in
a previous thread. Just to prove that method works, here are 9 noob archers training at the same time:
However, the best way to train marksdwarves is live fire -- you get more experience per bolt than archery targets. If you are lucky enough to get a forgotten beast who is unkillable by bolts, then you can use him to turn entire squads of dabblers into elite marksdwarves in less than a year.
First, build a room to trap it.
V- bridge covering entry to fortress
OOOOOOOBOOOOOOOOOOOOOOOOOOOOO
O/O.F_.....................OOO
O/O.F_.....................OOO
./+.F_.....................BBO - bridge to caverns to let beast in
O/O.F_.....................OOO
O/ORF_^....................OOO
OOOOOOOOOOOOOOOOOOOOOOOOOOOO
O = wall
. = open space
F = Fortification
_ = channel
^ = ramp (up from lower level)
/ = ammo stockpile
R = weapon rack (or armor rack)
+ = door
First, dig the entire area out, but do NOT breach the wall into the caverns on the right until everything is done.
Dig the channel in front of the fortifications, and remove the ramps. This stops the beast from standing right next to the fortifications, so it cannot shoot through them.
Build the bridges so they raise (not retract) to form walls. This lets you lock the beast in this room.
Link the bridges to two separate levers in a place like your dining room, where they will be sure to be pulled shortly after you give the order.
When everything is ready, set up a pasture in the middle of the room and assign a couple useless bait animals to it. Then send a miner to open the path to the caverns. Once he's done, have him hightail it back into the fortress, and close the bridge to the top wall.
The forgotten beast will charge in to nom on your bait animals; as soon as he's in the room, pull the lever to close the bridge on the right wall. This will trap the forgotten beast in this room.
Now all you have to do is make the weapon rack a barracks, and resize it so it takes up the 5-square room. Set it so your marksdwarf squad trains at it (press q, move cursor over rack, select squad, press T). If the squad is active and has no other barracks or archery targets, the dwarves will all pack into the 5-square room to watch biting demoonstrations or other useless crap. BUT instead, they will see the forgotten beast through the fortifications, and shoot at it. When they run out of bolts, they will try and leave the room, whereupon they will find a ammo stockpile, reload, and come back in. Note that it will consume somewhere between 300-500 bolts per marksdwarf to reach elite status, so make sure you make lots and lots.
My current fort has 23 elite marksdwarves, another 7 highly skilled ones that are close to elites, and an amber quadraped with over 3000 copper bolts stuck in its wounds.
This trick with barracks and fortifications is really useful. Set one up covering your front door, and you have archers at the ready. Just make it a pillbox with no door to the combat zone, so the dwarves won't go charging out to do melee when they run out of ammo. And since once your marksdwarves are elites, you don't want them doing archery practice (which wastes a lot of bolts), and you DO want them training shields or dodging, you want them training in the barracks you assigned them.