Hi Jim, a contractor friend of mine uses blue masking tape to mask just outside of the area you want to seal. After you're done applying the silicone simply pull the tape off leaving the area nice and clean. In regards to how to make a tidy straight looking seal, practice on a piece of cardboard. Also when you cut the tip off the silicone tube, make a small cut so you can better control the silicone. I like the small hand held toothpaste tubes of silicone they are easier to control and store. Just my .02