Get on the side railing next to Jabu's mouth. You want to be around the middle post of the three posts on the fence. After climbing up on it, just hold left on the joystick to jump towards Jabu's mouth. Time a jumpslash at the peak of your jump. If you did it right you should hit the loading zone and enter.
Sometimes you will get on the top of his mouth and slide but not enter. If this happens, don't give up. Keep jumpslashing when you're about to fall off and eventually you should hit the loading zone. The loading zone is slightly beneath you, so every failed attempt, try sliding just a bit further down to get closer.