Start thread and tie the large red dumbbell eyes on TOP of the shank near the hook eye, locked with figure-8 wraps and cement so the fly rides hook-point up.
At the rear, tie in a clump of craft fur for the tail/claws, about one shank long, and taper the tips into a wick shape.
Add a few strands of flash over the tail.
Tie in the Flex Hackle at the tail base and wrap it forward in touching turns to just behind the eyes to build the crab body; tie off.
Dub ice dubbing in a loop or a few clumps around and behind the eyes to fill the head.