- Use a fine brush. I use Winsor and Newton 000 for the finest edge highlights.
- Use retarder. This slows the paints drying time. Without it I can't move fast enough to get the paint onto the model before it dries on the brush. I use Vallejo retarder. The bottle says no more than 5% retarder. I have got up to 14% and it still cures, but this may depend on the paint you are using.
- Prep the brush. Wet it first. wipe enough water off that there is no longer a drop of water on the end of the brush separating the bristles. Get a tiny amount of paint on the brush and wipe off the excess.
Drag with the brush. Try not to stab with it. For some hard to reach areas this may be impossible. It's a good reason to assemble some parts after painting.
I like the strength of the edge highlight to vary. I think trying to get it completely uniform is difficult and does not look good. I usually try to get the corners a bit brighter. So I first touch the newly loaded brush near a corner, and then wipe the paint toward the corner.
The next stroke starts further back, but again wipes towards the corner.
And so on.
The only other detail about the technique is to suggest that doing several steps of edge highlighting with brighter colours is a good idea, particularly for large surfaces. Let me know what I missed, and how you do your edges in the comments.