A prerequisite skill for a back walkover would be to fall to a bridge from a standing position and preferable to kick over as well. For a front walkover I like to see that an athlete can do a front limber and stand without dropping their arms.
Walkovers, both forward and backward, are great skills for beginners to learn proper opening of upper back and shoulders to help prepare for front and back handsprings. A common misconception is that arching using the lower back and kicking hard will get the athlete through the skill. The skill can be done this way but it’s not recommended. The upper back stretched has up to 70% mobility allowing for it to be much more useful.
Some progressions I use are standing with athletes back to the wall, placing hands on wall and walking down into a bridge (they need to walk both hands and feet). Once the can walk down and up the wall I have them try to do it with one foot raised to build the necessary strength. It may take some time but will build the shoulders, core and leg strength needed.
I like to do what I call “tic tocs” as well. The athlete starts in a bridge, lifts one leg and kicks over back and forth only allowing one foot to touch the floor while hands do not move from the floor. This helps the athlete feel the shoulders open as well as hip flexor press at the same time in both directions.
For beginners who haven’t mastered the kickover yet, I’ll have them lay on their back with knees bent, place both hands overlapping behind the head and bridge up on elbows. This helps isolate the open shoulders (make sure they aren’t supporting weight on their head/neck). Once in the proper elbow bridge I’ll have them kick over. If they close their shoulders they won’t be able to make it.
The techniques of both front and back walkovers is fairly simple but the fear of going backwards is real. Be patient. In the front walkover athletes tend to want to rush to stand up, make sure the head stays neutral, shoulders open and really use the hamstrings and glutes to finish the skill.