Protruding your lips was probably easy to picture. Could you also picture what “rolling them in” means?

Here, “rolling in” means folding both lips inward over the teeth as shown below.

If rolling in your lips is difficult, first use your fingers to push both lips into your mouth, then make sound in that position.

Once you are used to the shape with your fingers, gradually release them. Practice until you can maintain it without using your hands.

There is no need to rush.

The difficulty comes from having used the muscles that return your lips to their original shape to stabilize—or rather, fix—your voice. Your throat should be able to make the sound on its own, but the lips have developed an interfering habit, as if saying, “The throat cannot manage alone; I must help!” This also held back the vocal folds’ development.

With repeated practice, you will learn to control your throat and lips independently, so take your time. The throat’s own abilities, weakened by this interference, will gradually return. With your lips protruded, practice distinguishing a thick Goo voice, a thin Paa voice and an intermediate Choki voice.