The first step in learning to tumble turn is to perfect the forward roll. Swim half a length, tuck your chin to your chest to initiate the roll and bring your knees in towards your chest in a tuck position. Once you've completed the roll, continue swimming in the same direction. When performing the forward roll, ensure you are continuously exhaling through your nose.
The second step is to initiate the forward roll close to the wall (how close depends on your size - but somewhere around the T) and push off onto your back, performing a dolphin kick or freestyle kick for 10 metres or so. Focus on planting your feet firmly on the wall with your toes pointing towards the water surface.
Once you've mastered pushing off onto your back, introduce the twist onto your front as you push off the wall. As you become more proficient, you can actually performance half of the twist within the forward roll.
Some pointers to help you perfect your tumble turn:
Keep your speed as you swim towards the wall. A good tumble turn requires pace.
Tuck your head in tight to your chest.
Continuously exhale through your nose as you complete the roll.
Aim to get your feet around 50cm below the surface of the water and shoulder width apart as they plant on the wall
When your feet have planted, stretch your arms out just before you push away from the wall, getting you into a streamlined position.