Grip the rings, tuck your knees to your chest and pull your hips up to an inverted postion.
Depress and protract your scapula, arms straight, core braced. Maintain a posterior pelvic tilt and keep your knees tucked in tight.
Slowly pull your hips through, rotating the rings so your palms are facing away from you. descend with control, lowering until your hips are level with your shoulders.
Stay engaged, & hold for time, stopping before form breaks down. Pull yourself smoothly back through the rings, by pulling the rings to your hips and further protracting your scapula.
It's a good idea to film yourself, or have somebody spot you, to check hip position as you develop your spatial awareness in the back lever position.