Start by hanging on a pull-up bar with palms facing out, or use the ab straps and hang with your arms inside of them. Use a box or bench if you need help reaching the bar.
Pull your shoulder blades down your back to keep your shoulders away from your ears.
With control, simultaneously lift both of your legs up and touch the pull-up bar. If you can't touch the bar, go as far as your mobility allows with control. Avoid swinging to isolate your abdominal muscles.
With control, slowly lower your legs back down to the starting position. This counts as one rep.