Having just given myself a crash course in edge highlighting, hopefully I can help you a bit.
You want your paints to be thin, but not watery. They should be thin enough not to chalk up but thick enough that they stay where you put them and don't run.
You want to make sure that you have enough paint in your brush to have some about halfway up the bristles, then lightly dab some into a paper towel. This is so that you can paint with the side of your brush, but you won't immediately smash a thick line on the edge when you lay your brush down.
Whenever possible, paint with the side of your brush instead of the tip. Try to hold the brush at such an angle that you can't hit either side of the edge. For an awful visual representation, if this is the edge: > and this is the brush: | you want to hold your brush against the edge like this: >|
When you are getting ready to paint an edge, hold your brush near the edge at the proper angle and very lightly take some test swipes of very light pressure, increasing the pressure until you know how much you have to apply to paint the edge. You may notice that you can actually paint an edge much too thin to really even see if you don't apply enough pressure, but obviously if you apply too much it's going to fatten out.
When you are painting the edge, don't try to do the whole thing in one stroke. Just do little bits at a time while trying to keep it even.
Perhaps the most important for me personally is making sure that I am holding the model in such a way that I have easy access to the edge, and can relax all of the muscles in my hands. I place the model in my left hand and rest that hand on the table, just loosely propping the model up. You will have to rotate the model in all kinds of different ways to get all of the edges. I place the pinky of my brush hand against the table, the model, or my other hand in order to stabilize it. That in particular is what really helped me improve.
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