There are a few schools of thought on this, so I'll tell you what I do:
1. I wait until fermentation has visibly stopped,ie.,krausen falls, no airlock activity to add dry hops.
2. I use nylon hop bags to keep the hop matter contained. If you can't find them, you can use a nylon paint strainer bag, tied shut. I add marbles to the bag, to weigh them down to the bottom.
3. Amount is more of a preference. I use 3 oz for an AIPA in the fermenter, and, being a hophead, another oz in the keg (using a nylon bag,etc.) when it goes in the kegerator.
4. I leave dry hops in the fermenter for 7 days @ room temp before kegging. Some guys dry hop longer, some less.
5. You'll have to experiment to find what you like best. Dry hopping rocks!