I take fresh live crabs and ice them down real good just until they're too cold to move, but still alive. You can also scald them if you like.
Remove the top shell, lungs and intestines. Break away the mouth and bottom side apron. Break the claws off. Keep them iced up.
Now
get your fish skinner out to do some clipping. First clip the legs off leaving just enough to be able to pull the leg if needed. Using the fish skinner allows for a nice clean cut. Trying to pull the legs off many times leaves you with a hunk of meat left on the leg.
Now
clip away the crowns of the shell. The crown is the part that stands the
highest if you were to lay the crab down on its belly. It's just a small
piece of shell but takes up space in the pot (and we don't want that do
we?) 
Next
take the skinner and clip away an upside down "V" shape in the shell
where the mouth was. The reason for this is that it makes it easier to
break the crab in half. 
Here's
what it looks like when it's clipped away. Now you should have crab bodies and claws.
The
next step is cleaning the shells to remove any dirt. Use an old
toothbrush (I keep one handy just for this purpose) and scrub the crab
body and claws to remove as much dirt as possible. 
Once
that process is complete break the larger crabs in half by folding them
inward and then back. Notice how easy they break apart because you
clipped away the "V" in the shell earlier. The piece you clipped away is
probably the hardest piece of the shell on the body. Break the claws in two at the joint.
That's it, you're done
They're ready for the stew or gumbo.
To freeze them put them in a labeled zip lock bag and cover with water. Get all the air out. Handle the bag carefully because there are many shell barbs that will pop the bag.