Instructions
Position a mirror against a wall or other sturdy surface. Make sure the mirror is roughly at eye level so you can observe your tongue as you try to roll it.
Stick out your tongue between your lips. Part your lips just enough so the tongue can come out. This will keep the tongue edges more firmly in place so the fold can be more clearly defined.
Bend either side of the tongue an upward direction, so that both edges turn up toward the center of the tongue. The edges should touch, or almost touch at the center, resulting in a clear line down the center of the tongue.