you just lie down and shoot your gun.

but that is how you do it.

harder way-regular layout- hold b to drop, while shooting and aiming/or hipfire if thier right in front of you.
easy and recommended way- tactical layout- tactical will switch your b button and right annolog stick controls so that b will knife and right annolog will drop. this helps for drop shotting so you can aim while you drop, and you dont have to move your finger to b to drop. the disadvantage is it makes it harder to knife, but who needs to knife when you can drop.