how can u do so many glute ham raises???
there isnt a GHR machine in my gym, so i improvised by putting my knees on one of those bouncy half dome things to cushion them, then hooking my ankles under whatever i could find.
cant even do a proper all the way to the floor one!! 
Well its not like im doing them perfect , lowering slow untill my nose touches the floor.
I am lowering with as much control as i can and when im about to colapse i use my hands!
I wouldnt say 3x12 are that many, but they are pretty challenging for me.
Heres Kelly Baggett's description for them ( covers your case where no machine is available ):
Natural glute-ham raise- Kneel down and lock your feet under something solid and
heavy (a partner can do just fine, but he must be able to hold you down). The trunk is
upright and the arms are alongside the body. Find a pad or something to rest your knees
on. A towel will work just fine for most. Without rounding your back Lower yourself
towards the ground as slowly as possible. To do so you must produce a powerful
hamstring contraction or else you'll find yourself embedded in the floor! If you're able to
bring yourself back up on your own, do so, but most guys will need a little push-off with
the arms to get moving.
