Get into plank position, pelvis tucked forward/under, abs tight. Elbows in line with shoulders.
Keeping abs tight, step one leg out to the side, as far as comfortable.
Now, bring opposite leg out to the same side, so feet are together. Reverse the motion, stepping one leg in toward the centre, followed by the other leg. Repeat on the opposite side.
[...] Plank Step Out [...]