You stand with your feet shoulder width apart and your hands clasped behind your head (or resting on your shoulders, with your left hand on your left shoulder and right hand in the right shoulder). You bring your left knee up as you bring your right elbow down to meet it, then straighten up again. Then you bring your right knee up as you bring your left elbow down to meet it, and straighten up again. You don't have to actually touch knee to elbow if it's too hard (but it will get easier), the focus is mostly on bringing the knees up as high as is comfortable and creating the stomach crunch with your elbow and upper body movement. If you're doing it with your hands clasped behind your head, remember to not actually pull on your neck during the exercise. I find them to be easy and effective.

I wish I could do more squats, but half a lifetime of lifting in the workplace with my knees means squats kinda hurt after a while so I do minimal amounts of wimpy not very low squats.