Firstly you obtain monkeybombs and then after you run out of all three you will have four grenades left.

You throw a grenade on the ground and when you are ontop of the grenade it will say Press RB to throw back, when it says this you press LB and you spawn a monkey bomb out of nowhere.

Note: The grenade will still be on the floor and you can down yourself if you dont have juggernog.

Extra 1: If all four players have monkey bombs one player can throw a grenade and all four people can spawn a monkeybomb. This results in a total of at most 80 monkeybombs. In this time you will most likey get a maxammo and you will have full monkeybombs.

Extra 2: You can buy extra grenades from the teleporter room in kino and the zombie experimentation room in Five.