This is probably one of the hardest exercises to work your core as it focuses in on both the top and bottom portions of your abdominals. With the ab wheel on the floor, get on your hands and knees and grab the handles of the ab wheel and extend straight out so that core is parallel to the floor and then return to the beginning position. Like most bodyweight exercises, you can this one slightly easier or harder by pulling your knees off the floor or keeping them on the floor. To work the obliques, you can curve the path of the wheel slightly to the right or left and target those sides.
Ab wheels come in different sizes. The one from the video below is a bit larger than many of the smaller ab wheels you'll
typically see in the gym, but they all work the same way.