There are several ways.

The "traditional" route is to study Crab at Crustacean U.

Another popular method is to attend a Crab bootcamp where they teach you the basics like walking sideways and dealing with eye stalks.

Lastly, I have known many self-taught Crabs. They're usually starting out as crab-adjacent crustaceans but have known at least one former barnacle and two former prawns.