Given that you’re interested in this backbend, my assumption is that you’ve got Urdhva Danurasana in the bag (!) with a nice lifted chest, arms approaching straight and a backbend that incorporates the whole spine, not just the lower back.
Seeing as that’s the case, start your exploration of Eka Pada Urdhva Danurasana (one-legged wheel pose) in Urdhva Danurasana. Grip the floor with the fingertips, work the feet in parallel (because it creates more space in the lower back by encouraging you to engage your quads rather than glutes), push the feet firmly into the floor (because it helps to lift the hips higher) and reach your heart back and up through the shoulders.
See if you can shorten your base slightly by walking the hands and feet a little closer. If you experience any compression in the lower back when you do this focus on lengthening your tailbone towards your knees. If that doesn’t help then walk the hands and feet back to their original position.
Now, step your left foot a few centimetres in toward the midline of your mat. This doesn’t need to be a big adjustment, just big enough to centre you once the other foot lifts off the ground.
Test your strength and balance by lifting the right foot a few inches off the ground. This might be enough of a challenge and if it is, great, stay here. If you’ve got a little more wiggle room, bend your right knee and lift it as high as you can. Imagine you are trying to touch your bellybutton with your knee.
Pause here and take stock of your the form of your backbend. Make sure both hands are carrying the load evenly, that your supporting leg is strong and steady, that your lifted leg is parallel (not turned out) and, importantly, that your hips are still lifted high. Commonly, the weight of the lifted leg will push the hips down towards the floor. Don’t let that happen! Instead, send a mental note to the supporting leg to push the floor away, and then…
Straighten your right leg! You may not get all the way there at first but that doesn’t matter. Just keep lifting up through your toes (as though someone has you by the top foot and is pulling you up off the floor) and continue pressing firmly through hands and foot that are still in contact with the ground.