1. Use Masking Tape
In order to keep your lines clean, you can use masking tape to mask off the areas where you want to separate colors. If you are going to apply the masking tape over the top of a base coat, just make sure the base coat has cured overnight, or the tape will pull it off. Also, don’t leave the tape on for longer than about 30 minutes or it will pull the base coat off.
2. Use a Small Paintbrush
Another alternative to using masking tape over the top of a base coat, that I prefer is using a small paintbrush. I have found that I can get a package of decent, small brushes at Michaels for around $6 with their discount coupon. The paintbrushes are great for getting in tight areas and making clean lines.