Right first you need a good enamel, thats got good consistency........not agloopy one or a runny one...................
Creative enamels work brill................
Like Cazz said, one fluid movment across the free edge..............
rest your brush hand on the edge of the desk, this will help to steady your hand................
A great way to practice would be, and i have pinched this bit of advice from the L&P section on french smile lines.....................
Take a marker pen, draw a smile line on the back of a tip, then try to follow the line on the front of the tip with your enamel........
Practice makes perfect smile lines..............this way you get a feel for the brush, the amount of enamel needed.............
hope this helps
love Ruth xxxxxxxx