Steam or boil your crab at ~7-8 minutes/pound, lid on but cocked to allow a bit of venting. I chose to steam mine because I've got a large steamer insert, and steam decreases the amount of water retained in the body cavity. Boiling will get the job done quite well, though, and more people are likely to have a big pot than a big steamer.
If you're adding a large number of crabs, start your timer when the water comes back to a boil. Base your number of minutes on the average crab weight, not the sum total.
Round times down whenever necessary; raw/undercooked crabs can always be cooked further, while overcooked rubbery crab is unsalvageable. Thankfully there's a decent margin for error in crab-cooking.
If you're boiling, you can add some seasonings to your water. Add salt (or use sea water), a bay leaf, some celery chunks, a carrot, some pepper corns, and any other whole spices you've got sitting around. Pre-mixed crab boil seasonings are available, but not necessary.
NOTES FROM THE COMMENTS:
- Take advantage of their cold-bloodedness! Chilling the crabs in the fridge or freezer just before use will slow their metabolism down enough that you can handle them easily.
- You can clean your Dungeness crabs before cooking. This keeps the mess out of the pot and allows for more crabs in the space. Just pre-chill your crab as described above, follow the instructions from steps 3-9, then cook according to this step's directions. However, this approach does require vivisection, which freaks some people out even if the crabs aren't moving. Take your pick. Also, if you've pre-cleaned and halved your crabs, re-weigh the parts (average weight of each half-crab) to determine cooking time.
- many countries/states require you to take your crabs home before breaking them down, so they can be checked for legal size by any passing fisheries officer. If you break them down while still on your boat, it may be construed as an attempt to sneak off with undersized crabs.