Buy some thinner also, get a little ceramic bowl. Mix a small amount of paint and thinner together in a ratio of 4 drops thinner to 1 drop paint. Dip your brush into the mix, dab it on a piece of paper to drain away excess then dab the brush into a numeral cutout. The mix will run to the cutout limits, aid it along with the brush gently. Fill up the marker spaces. The thinner will evaporate out and leave you with very smooth level markers. Use a damp (not dripping) q-tip with alcohol to clean up excess on the insert surface before it dries. Don't let alcohol get into the cutouts. If some dries do the same but use thinner or acetone (goof off)
Do more coats til you're satisfied with it. Keep stirring your mix as you work. About a teaspoon of mix will do all the cutouts.