First you have to get a kill streak of four (three if you have hardline on).

You have to hold it like you would cook a grenade, hold down RT.

Climb up on some thing so 'A' will apear. Climb up still holding it down, then quickly press right on the d-pad and throw it (if it is ammo you will not be able to get another).

If not you will get infinity airdrops.