For practicing this trick you need someone to play with, the wall is also possible.
Step 1: Make sure that the ball will be played to your favorite foot (or shoot first against the wall and make sure the ball comes back)
Step 2: Do a stepover with your left foot
Step 3: Flick the ball up with the inside of your right foot
Step 4: After that flick the ball with the left foot across your opponent
Cross-Over Video Tutorial