To do this, you'll want to hold the crab in the palm of your hand with the crab facing you (it will try to fight back). Place the thumb of your hand that is holding the crab over the part of the claw that connects to the rest of the body for leverage and to keep its claw from reaching back at you. Then, with your other hand, place your thumb underneath the pointed edge of the crabs she'll above your other thumb. Rest your hand on top of the rest of the shell and simultaneously pull the shell up and push the crab down with both of your thumbs (being careful not to separate the claw from the body).
Another way to rip off the shell is to lay it on a flat surface and grip its back four legs with one hand, and ripping off the shell with the other hand in the same way described above.
The shell should come off in one piece. If it doesn't, make sure to check that not bits of shell fell into the crab. It might be a good idea to remove the shells from all of your crabs at once to allow them to calm down after losing their head. This also gets all of the crab fighting out of the way first. This step gets easier with each successive crab and cleaning.