The turn is basically a somersault and twist. Here's what you need to do:
Swim towards the wall
Forward somersault just before the wall
Place feet on wall & stretch arm out
Push hard on wall and twist onto front
To try to help you learn and improve your tumble turns here are some ways to practice the turn and also five top tips for a good turn.
There are a number of different ways to practise.
It will help you to improve the turns if you practice different parts and then put it all together.
Here is a good way of doing this:
Practise pushing from the side and doing a somersault
Now try swimming five strokes and then doing a somersault
Now swim towards the wall and do a somersault putting your feet on the wall as you go over.
Now practise pushing away from the side underwater and streamlining (flat and thin with one hand on top of the other)
Once you can do this try adding a leg kick and then starting to pull as you reach the surface.
Finally, put the whole thing together - swim, tumble, push off, streamline and swim