Doing under the chin is an advanced maneuver and personally I wouldn't even attempt it until you have a good bit of shaving experience.

However here is what I did to master it, nothing. I shaved for weeks and weeks and when I came to that area I felt all thumbs and finally gave up. One day while shaving in that area I tried it and it just worked. The moral of the story is as you gain more facility with the razor the more challenging areas become very easy. You just need to give it more time and not force it.