Rigging half a blue crab is a very popular way to fish for redfish and black drum as well as many other species. You first need to remove the claws, if you let one of those guys get a hold of you it will not feel very good.
To do this you will need a pair of pliers, with pliers in hand grab the base of the claw next to the body of the crab. Sometimes the claw will just fall off, but most of the time you will have to squeeze the pliers and bust the shell to remove it.
Next you need to cut the crab in half. To do this you need a good sturdy fillet knife, don't try to attempt this with a flimsy knife.
Once the crab is cut in half you need to insert the hook from the underside of the crab in between the two rear legs and push the hook out the top of the shell.
On the top of the shell above where the hook was inserted you will see a light colored, almost white spot. This is where you want your hook to exit the shell.
Move your hook back and forth while pushing through the top of the shell so it does not break the shell or make a large hole.
Busting the shell or making a large hole will make it difficult to keep your hook in place causing the crab to possibly get fouled on the hook making it very likely that you will miss your fish.