Here's how I do it, and it hasn't failed yet.
Step 1: Appoint an arsenal dwarf.
Step 2: Produce steel gear. One order of gauntlets and boots produces a left and a right, so just make one order of each. So, for each dwarf you intend to have in your military, produce one order of:
* battle ax
* shield
* helm
* greaves
* mail shirt
* breastplate
* gauntlets
* high boots
Step 3. Create a uniform. I tend to call mine FullmetalAxedwarfist, but the name doesn't matter. You can get here from the military menu through [ m ]ilitary-[ u ]niforms. Assign one order of each item in the previous list, and change their [M]aterial to Steel, and change Partial Matches to Exact Matches, and make sure the Wear Over Clothing option is active.
Step 4. Create a squad that wears your uniform.
Step 5. Make absolutely certain no merchants, diplomats, or anyone else that might possibly wear steel armor is on the map anywhere. Otherwise, your arsenal dwarf may assign someone else's steel equipment to your dwarves.
Step 6. Add one dwarf to a squad assigned to your uniform.
Step 7. Let him get his equipment before adding another dwarf.
Step 8. Repeat steps 5-7 until your military is done to your satisfaction.
Step 9. DO NOT EVER SET YOUR DWARVES TO ACTIVE.
Step 10. Build a barracks for your dwarves, and enable the individual and squad equip, as well as training.
Step 11. Only give orders to your military in times of emergency. While not on-duty, they'll be performing individual combat drills in the barracks.
This is the system I've used, and it hasn't gone wrong yet.