My preference, regardless of whether the lettering is meant to be read, or not, is to wait until the painting is dry, lightly apply some oil/solvent medium (I use 1/2 and 1/2 walnut or linseed oil, and OMS) on the area you want to put the lettering. The type of brush depends on the type of lettering.
Usually lettering needs to be precise and centered. Often light guidelines are helpful, both for spacing and perspective. One thing that a typesetter needn't worry about is perspective!
I just finished this one:
I oiled the face of the rock, used a straightedge to keep the lettering fairly straight, but choose to otherwise 'wing it', since the letters were not on a flat surface and were chiseled freehand. It came out satisfactorily, although compared to the photo, the words did not quite fill the rock as much. However, I doubt anyone is going to hold up a photo next to the painting, nor drag it out to the site for comparison.
If I can find it, I will post an image of a painting I did a few years ago of some small bottles of hoodoo oil used to dress candles.