Sit on the floor with your elbows resting on a bench (or chair, etc.) behind you and a booty band around the bottom of your thighs. Make sure you're seated tall, lengthening through your spine. Do not slouch.
Open your knees to either side, pushing against the booty band for resistance as you do.
Go as far as your hips allow without causing pain, then bring your knees back together. This counts as one rep.