Assume a prone position, on an elevated surface such as a bench. Legs off the end of the bench, so you can bend your knees and hips at 90 degrees to start.
Keep your spine neutral, and get into posterior pelvic tilt, grip the bench and create tension in your upper body.
Extend your hips and raise your legs to parallel. Pause at the top of the movement, leg, hips & shoulders in line. Avoid raising your legs past parallel as this will place excess strain on the lower back.
Lower with control. Repeat for reps.
Note : Pick the version which suits your current goals. Eg. Straddle will translate best when working on Front or Back lever straddle. As you progress to Full Levers, work on Full Reverse Hypers.
Remember to extend at the hips not the lower back.