Here is how I steam my 3 lbs live Crabs:
Rinse the crabs in water.
Arrange them in a big pot.
Pour in 1/2 cup water or 7up.
Start the flame in medium low heat.
Make sure to cover the pot and steam for 15 minutes.
You may add a little more water if you think the crabs are not fully cooked yet and steam more for few more minutes.
Careful with the heat. Keep checking every few minutes, you don't want to burn those yummy crabs.