put your thumb nail right at the back end of the crab, your find a lip there where the shell over laps. hold the legs down or bottom of the crab and lift the shell up from the back to the front.
once the hard shell has been lifted cut the crab straight down the middle, its your call wether you leave the legs on or not, I like to or at least a couple legs as it looks natural in the water. If its distance your after, cut all the legs off but leave the back paddle on and rap your bait cotton all the way down the body of the crab to the paddle so its shaped like a torpedo.
The lemons caught above are pretty small ones but would have no trouble what so even smashing back halve a big crab. 5/0 circle hooks insure very little escape the strike. and if you struggle to bait up with on hook use a second sliding keeper say 2/0 circle to hold it in place. dont be stingy with the bait elastic,
use the leg holes to thread your hooks through, take the time to place your hooks with plenty of barb exposed as most fish will run with it in there mouth before before they deal to it.
bait presentation is important.