Mount the hook, lay a thread base, and lock a dumbbell eye on top near the eye so the fly rides hook-point up.
At the bend, tie in two small splayed clumps of craft fur or fur as the crab's claws, plus a pinch of flash between them.
Bind in two or three pairs of rubber legs across the shank, splayed to the sides so they wave like walking legs.
Sandwich the shank between top and bottom furry-foam pieces (or fold one piece), bonding them with flexible adhesive to form a flat, durable carapace around the hook.
Trim the foam into a rounded crab-shell shape that keeps the hook gap clear.
If desired, tie in and bond a short mono weed-guard loop at the head, then whip finish.