Pretty much the best explanation I could think of for learning the backflip was in that video, which was learning how to ride a bike. If you're already working on your backflip, you should know that muscle memory is important and of course, we're using "muscles are body isn't used to using" in that particular way. So the only progression I can think of is:
1. Stand on a soft and protective surface, preferably a mat.
2. Have a spotter use complete and "heavy" spotting to help you get height and rotation for your flip.
3. As you gain the ability to land more often on your feet, have the spotter spot less and use only one hand for slight support.
4. When you're ready, attempt flipping without the spotter. Usually when you can land the backflip more often than not.
If you have something as soft and thick as a crash mat, you shouldn't worry about landing and hurting yourself and especially with a spotter, the most you can do is land on your back or knees. If you don't have a spotter, there's no progression steps except either landing it or not landing it, imo.
Personal tip: I realized the reason I was always landing on my knees was because on the first several attempts on doing a backflip, I was not tucking hard enough and bring my arms down to grab my knees in, rather than my knees coming all the way to my torso and arms locking them in. So use your abdominals to really charge your knees up!! And if it is your first attempt, you are most likely not going to land it unless you have absolutely no fear or doubt of going backwards.