To do Swimmers, you will lie face down on the ground with your arms out straight overhead on the ground and your legs out straight behind you. Everything should be relaxed and your neck should be in a neutral position.
Then squeeze your glutes and engage your upper back and shoulders to raise your chest and legs up off the ground. Try to get your chest up as high as possible as well as your quads. Keep your neck in a neutral position. While your lower back will work, make sure you’re using your glutes so your lower back isn’t having to carry the entire load.
Then, holding at the top, being to flutter your arms and legs as if you are swimming. Raise the right arm and left leg up a bit higher then lower them as you raise the left arm and right leg up. Try not to rest the opposite arm or leg on the ground as you flutter. Try to keep your chest and quads up off the ground.
Keep the movement small and focus on engaging your backside!