FACT: I don't own a large pot with a steaming basket.
I've taken my large pot, filled it with approximately 2 inches of water, then placed an oven-safe glass bowl on top of the water. Make sure the water isn't near the top of the bowl because you want to steam the crab, not boil the crab. Nothing ruins your taste for crab like breaking open a leg and having scalding water drain out. Here's my MacGyver creation of a large steam pot:
While you're waiting for the water to boil, place the crab legs (I used snow crab legs) onto a plate and season them generously. My crab legs come wrapped in butcher paper, so I arrange the crab legs on the paper and sprinkle them with Old Bay seasoning.
Once the water starts boiling, turn the flame down to simmer, put the seasoned crab into the bowl and cover the pot. Cook for approximately 15 minutes.
Use tongs to pull out the steamed crab and enjoy. Some people dunk their crab into melted butter. That's not how we roll.