There are usually a number of solutions in every level, so take your time and think of your own.

Do a couple of trial runs first, just to see how everything behaves in the level and to get the feel for it.

If you don't think that you need to transform into an animal to complete the level, it might still be necessary to get 3 stars.

In order to unlock new worlds, you need to get a certain amount of stars. If you feel you can't move forwards anymore, go to an earlier level and get some more stars.

Use the bubbles to lift up objects or even Om Nom in his original form.