You can do this seated, kneeling, standing, etc. Pick the best position for your body. Set your core and stabilize your shoulder blades. Hug your center line (you can always place something between your legs if you need assistance in firing your inner thighs-just make sure you don’t lose your alignment).
This exercise is exciting because you are working the 3rd head of your tricep! What’s the secret? Keep your elbows behind your back!
Variations for the arms-you can try single arm, controlled pulses and reciprocal (one arm is moving in while the other arm is moving out).