1.) Use your core/stomach muscles to drive your flip
2.) put your chin to your chest when beginning the flip. Do this with some force, as it really does help to get your momentum into a flip/tumble motion
3.) pretend you are just doing a somersault in the water.
4.) "land" the somersault on your back, pushing off the wall while you are still on your back. You can twist from your back to your stomach during the push off
5.) One drill you can do that helps a lot is the "flip turn without a wall" drill. Push off the wall in streamline like you would normally do, and then swim 5 increasingly faster strokes. On your 5th stroke, do a flip and "land" your somersault on a pretend wall in the middle of the pool. Yes, you will have lost your ability to continue swimming freestyle at this point, but that's not your focus! Your goal is to do the flip "turn" very smoothly, with chin-to-chest, a completely straight-up-and-down flip (not leaning to one side or the other), and using your last stroke into the turn as a "catch pad" to flip over.
You can also just practice somersaults out of the pool on a soft surface somewhere in the gym or at home.
And watch others in the pool do them. Ask a coach to help you if there's one available to you. But really, practice does make perfect. Keep trying, don't give up!