NC;
First off, thin the cresting paint enough so it will not create a "gob" on the end of the fibres. The paint has to be thin enough to flow from the brush. When you look at the brush you should only see the fibres, not a thickening of the fibres.
Use good quality brushes, I like silver fox for water based paints, and sable for Testors.
Make up some sort of guide, to attach to your crestor.
I made one out of rod, and brazing rod.
I have a rod from side to side on my crestor, on that rod are bigger diameter rods, about a half inch in length, with a piece of brazing rod brazed to them, and sharpened to a point. They have a set screw in the back of the bigger rod combo. Then you just slide the bigger rod combo, up and down the side to side rod, and hold it in position with the set screw.
If some one would help me, with posting a pic, that would explain it alot better than my words can.
Make sure your arrows are straight.
Also use a good quality crestor, hard to get mint lines with a sewing machine motor.
Good light also helps.
I also like my crestor to rotate away from me, that way I can see what is happening as soon as I touch the brush to the shaft.
I also like to brace my hand, so I'm as steady as I can be.
Hope that helps
Jim