This is one that I find harder to teach little ones. A lot of time it comes about naturally as they spend more time on their tummy, and also begin to push up more onto their extended arms. This is why it is really important to encourage tummy time and prone skills.
To work on this skill, have your child spend a lot of time on their tummy. There are lots of ways to make this easier and I have linked several ideas throughout this post. As they spend time on their tummy, the will begin to prop on their forearms. From here they also learn to push up onto extended arms. While they are learning these skills they will also start weight shifting (moving from side to side) from one arm to the other. As they are refining this skill and getting stronger it looks like they are just finding stability by swaying from side to side.
It is this weight shifting and attempts to keep their head and chest up off the ground that help with rolling onto their back. Initially it looks like they plopped over. If you look at my video in my last post you will see part way through how he is on his stomach and lifting his head and chest and just rolls over. That is generally what it looks like in the beginning, that they ‘fall’ over onto their back.
For kids that you are trying to help along with this skill, you can use a toy to engage their attention while they are on their stomach and then help them weight shift so that their weight carries them onto their back. This becomes more controlled and refined as they begin to practice and learn how to control their movements.
Another thing you can do is have them work on sidelying. From this position it is a lot easier to move onto their back. You can start with this and use a toy to have them follow with their gaze so they complete the roll. Yes this is only half a roll but you can increase it by having them almost lying on their stomach (maybe have them half lying on a pillow so it is like they are lying on an incline) and then they are adding more and more range to their rolling.
These are a few ideas but the biggest factor is going to be getting them to spend more time on their stomach and work on weight bearing on their arms and weight shifting.