Start in side plank. For a more gentle version, go down onto your elbow. Either way, make sure lats are lifted away from the floor and contracted towards each other. Your feet should be very slightly out in front of you, so when you lift your hips you can maintain your shoulders stacked over each other (top shoulder will want to lean back or way forwards) as well as a pelvic tuck (low back will want to arch). Lift your top knee towards your elbow; it doesn't need to touch. Focus on keeping abs contracted and pelvic tucked and only lift as high as you can without losing form.