Standing ab-wheel

Stand with your feet shoulder-width apart, hinge at your hips, and place the ab wheel on the floor in front of you with your core braced and lower back slightly rounded.
Roll the wheel forward under control, extending your body as far as possible without letting your lower back arch or your hips sag.
Contract your abs and lats to pull the wheel back toward your feet, returning to the standing position.

Tips

At the start of each rep, round your lower back slightly and tuck your pelvis before rolling the wheel forward, so your spine is already in a stable position before the load increases.
On the return, think of pulling your elbows toward your hips rather than just pushing the wheel back, which helps you use your lats to assist the abs through the full pull.
Roll out only as far as you can maintain a stable spine throughout the movement. Stopping short of your maximum reach is correct technique, not an incomplete repetition.

Allowing the lower back to arch as the wheel rolls forward, so the lumbar spine extends instead of staying in the rounded, stable position established at the start.
Letting the hips drop toward the floor during the rollout, so the body sags into a diagonal line rather than moving as a single unit.
Using momentum to snap the wheel back on the return instead of pulling it back under control, so the abs and lats do not complete the movement.