It's as a partial backwards somersault - i.e., let the roll continue a bit onto your back. I leave my legs mostly straight, so it doesn't have the speed to become a full somersault. It's very easy and gentle. The biggest issue with falling backwards is if you don't tuck your head in, and you fall too fast, you could hit the back of your head hard against the ice. So leave the legs fairly straight, and tuck that head. Another issue is that some people try to stop the roll by sticking out their elbows - which isn't a good idea.
If you are nervous, try falling at home, without skates - on a rug, or on something soft, like your bed, at first.
If you've already injured your lower spine, and have been told not to place weight on your lower vertebrae, maybe you could roll off-diagonal, so you roll a bit onto your hip, another very gentle fall. That's also safer if you must also fall on rugged terrain, where something sharp might hit your spine.
At Fort Dupont Ice Rink, we teach all beginning skaters to fall. (We usually start falling and getting up off-ice for the first couple minutes.) Some of us teach them to fall in all directions - forwards, sideways, backwards. The young students are almost all totally unafraid of falling, and they love it. Some of the older teenagers can be a bit more hesitant.
But I ran into a problem last session. Many of the students in my Snowplow Sam 1 class got bored with repeating the same skills after the first day, which they all pretty much mastered that day. (They were all young and mostly fairly athletic.) So they all deciding to fall down and do snow angels on the ice, and refused to get up.
I tried using stuffed animals (balance and throw games, which kept their attention for exactly one class), games, teaching more advanced skills, etc.
But they loved conspiring to annoy the instructor and staying on the ice. If I pulled one of them up, they promptly chose to fall down again.
I'm curious what real instructors do in situations where all the students conspire to ignore them.