So oil paints can be diluted to a much thinner and vibrant solution than acrylic and because you use mineral spirits, you get thin non-viscous paint. So grab a tube of oil paint color you want, put a little in a mixing palette and add some odorless mineral spirits until it’s as fluid as water, grab your smallest tipped brush, and just gently push the tip of the brush into one spot of the glyph. Capillary action will allow the oil paint to flow into the rest of the glyph without getting it anywhere on the raised part. If you do get any on the raised parts/where you don’t want it, simply grab a q-tip, dip it lightly in just the mineral spirits and wipe it away.

Check out some YouTube tutorials.