These are some great exercises for your abs. I’ll go over several ways to do a situp (crunch) both with and without a bench. Equipment needed is an exercise mat and a bench.
When doing a situp, lay flat on the mat with your knees bent and about shoulder width apart. Raise up into a crunch and squeeze your abs. Slowly lower back down. You can either rest your hands gently at your ears (but don’t pull on your neck) or you can lay your hands flat at your sides to ensure your abs are doing 100% of the work.