Flutter kicks are harder than they look (trust me).
Lie flat on your back on a mat with your legs straight and your arms extended alongside your hips, palms flat on the mat.
Lift your legs about 6 inches off the ground.
Begin the movement by lifting one leg higher than the other. As you are lowering that leg to starting position (6 inches off the floor), raise the other. This should be a fluid fluttering (or up and down) motion. Your feet should never touch the floor. This keeps tension in your core.