This has been covered a lot so look at past posts but here we go:
1) make sure rack is ample distance from bottom of pot ( at least 2 inches)
2) have secure lid
3) Fill bottom with about .75-1.25 inches of water ( yes you can add vinegar or beer) personally I like to drink my beer.
5) Get a heat soure that can adequately keep a full rolling boil and keep pot very hot
6) Heat said pot untill hot to touch and full of steam
7) Load pot with crab, leave a little room at top, use a weight to keep the buggers in.
Steam for 30 min
9) set on well papered table and dig in. Those who refuse to take the shells off there crabs don't need to eat and should bring more beer.
Typically you know they are done when a little fat comes out of them and the apron is a little loose. You have steamed to long it all the claws fall off or if they become very mushy. Watery crabs are often indicative of light crabs or crabs that have come in contact with water from the bottom of the pot.
An interesting fact I learned this weekend from procrabber is that it only takes about 7 min to steam a crab the time lag is the result of having to adequately heat the surrounding air. The other crabs often absorb the heat. This is why one should have a good heat source.