Draw the yarn through the first edge stitch just like you would if working a single crochet edging.

Chain 1 stitch.

Take your crochet hook through the stitch to the right of your first stitch. (This may feel a little bit weird as normally you work from right to left with a crochet edging).

Draw the yarn through the edge stitch, just like a normal single crochet stitch.

Wrap the yarn around the hook once more.

Draw the yarn through both loops on the hook. You have completed your first crab stitch!

Keep working in this way from left to right across the edge of your work to make your crab stitch edging.