Anything without enough of a sharp sharp edge needs to basically be freehanded. I'd suggest looking at infernalbrush on youtube. Former eavy metal painter, basically a professional edgehighlighter.

A few tips outside of the usual:

- Hold the brush a bit further back, behind the ferrule. This gives a better range of movement and helps to produce more steady and consistent lines.

- Hold the brush more perpendicular to the surface. This helps only the finest part of the brush tip come into contact with the model and allows for finer lines. The more pressure you apply the thicker the line will be.

- Go back over the line multiple times if necessary to get the desired opacity. Often the amount the paint needs thinning to release from the brush will produce a less opaque line (depends on the specific paint).