I position a bench under the bar and step up on to it after each eccentric rep so I am back at the top of the movement. Don't rush or jump too vigourously to get back to the top or you'll end up swinging all over the place. If you hang a weight from your waist then you need to ensure that the bench is not in the way as you lower to full extension or the weight will hit the bench, effectively removing the extra resistance.
Take about four secs to lower for each rep to begin with. Stop when you have completed your allocated reps or when you feel that you can't control your downward motion. HIT used to suggest about 8 seconds for each negative rep to begin with, but the loads were lighter, and at that tempo it's more like a static hold than a smooth lowering motion.
If you find your grip is causing you problems just use lifting straps, but if you can avoid them so much the better as your forearms will then get a good work out too.
All pretty straightforward stuff really. Hope it helps. Let us know how you get on with them.