Below are two common ways to fold a pocket square. Select the choice that you feel best fits your sense of style.
Grasp the square in it's center and allow the points hang.
Slide your bottom hand down and pinch at the center.
Fold the hanging points up behind.
Tuck into the pocket and adjust it to your liking.

With the points up, grasp the square in it's center.
Slide your other hand up and pinch at the center.
Fold the bottom half up and behind
Tuck into the pocket and arrange the points to your liking.