Leg rises are simple to do and don’t require any assistance of a machine or weight. All you need is a reasonable space around you and a mat.
Lie on the mat on your back and position your body in a straight line, with a straight back and head looking up in the ceiling. This exercise is performed by lifting the lower body in the air with straight knees and upper body stationary. Place your palms on the floor to maintain the upper body straight and the lower body moving up. With that initial position, lift the lower body up with straight knees to about 70° degrees; then lower them back to initial position but don’t touch the floor. Then make another repetition.
Make the first set as much as you can, then the second set if you are capable make the same number of repetitions or make a few less. The key is to challenge the abs to contract strong, so do as many repetitions as you can in order to trigger that contraction. Exhale when lifting, inhale when lowering.
This is quite a strong exercise so don’t rush into many repetitions at once. Let the abs do their work one set at a time.