Again, it’s really important to warm up the spine before entering full wheel. You can do this by taking a couple of rounds of bridge pose. Cobra, locust, and camel pose are also good backbend options.
If you’re having trouble getting into the full extension of the pose, practice getting to step No. 4 and holding there. You can also begin lying on a bolster and work on getting your arms into position. Focus on pressing hands and feet into the ground, until you’re strong enough to push yourself up.
To maintain proper alignment, try squeezing a block between your thighs or placing a strap around them to keep your knees from splaying out.
And lastly, backbends are best in sets of three, so try to do this three times (or two sets of bridge and one full wheel).