Crab baits can be split into 3 categories:

Hardbacked Crabs

Most larger fish when gutted contain crabs in their guts, but as a bait I'd only use them to target 2 main species, those being wrasse and smoothound. The latter if i could get no, or to bulk up a poor supply of, peeler crab.

There are several ways of hooking them but you want to keep them in one piece and fairly lively hence I'd use a suitable sized hook and insert it through one rear leg socket and out the opposite one, a little elastic binding will be handy for distance casting.

Hermit Crabs

These are the soft bodied ones usually found living in an old whelk shell, can be collected or bought frozen. If frozen use quickly as when defrosted they will turn foul & unfit. To hook, 1st evict crab from whelk shell & mount on hook/s, (pennels are good). This is a very soft bait & will definately need elastic, the fish often just snatch the soft body from the shelled head end so make sure there's a hook here and not just 1 hook at the head end or break off shelled section and just use the soft body.

Peeler Crabs

Peeler crabs = crabs that need to shed their shell in order to grow bigger, usually once a year. The females tend to moult 1st then the males a couple of months later. This is triggered by water temp, i believe, and happens in spring and summer, though the season is extended in the south west of the UK.

Most fish will actively seek peeler when the moulting season is in swing, though they are mostly used to target bass, cod and smoothounds.

Some tackle shops will stock fresh peeler when in season but usually to order as there is a fair demand + its damn expensive. Frozen peeler crabs can also be bought but the quallity varies so look to avoid any that have gone black. Once defrosted they go stale quickly. Soft shelled 'chinese mitten crabs' are availiable frozen but imho are a poor alternative.

To Hook

Remove hard shell & legs, cut to size or use whole mounted on a single or penneled hooks, then bind with elastic as its a soft bait. Ensure hook points are not masked. The legs and claws can be peeled to add to hooks or to tip a worm or fish bait.

Top Tip - Don't forget to set the drag on your reel if smoothys are on the cards, every year some one looses their rod and reel coz these fish often hit & run ask Lee Graham.