I tend to draft 2 from the first immigrant wave. They are supplied a shield and a helm, and train in wrestling. Later immigrant waves bring my military total to 10. They are started in the same fashion. Each dwarf remains a squad to themselves.
As they devleop armor user and greater stats they are supplied more armor. When I can make decent full sets of armor and silver war hammers they get switched to hammer and are carefully decked out in good armor.
By the time they achieve legendary with the hammers I have a sizable stockpile of bone/wood bolts. I set them to crossbows.
Defeating a siege tends to only require 1 or 2 of these fine soldiers. The moment the siege appears I call all of them to active duty and station them at the gate. The first two to show are dispatched to the siegers. Most of thier kills actually come from hammering the siegers with the crossbow.
I don't use stockpiles for my practice bolts, and don't produce any metal bolts. Once all the military has reached legendary in marksdwarf I stop bolt production. I let them use up the entire supply. After a siege I produce a few stacks of bolts, and once they have refilled thier quivers I then unforbid all the fired bolts. This causes the military dwarves to run out, grab them, and shoot them at the ranges.