After 30 years model building I have found several methods to do this neatly but masking of the edges is unnecessary and paint will nearly always find its way under the tape in a few places. A steady hand and confidence is the best method. The paint has to be thinned so that it flows easily into the edges.

1) use the correct type and size of brush, make sure it has a square end and is long enough to carry sufficient paint to enable you to finish an edge with one smooth brush stroke. you do not want to go over the edge repeatedly as it will show the brush marks if you are using matt black.

2) An good quality air brush will make short work of it but you do need experience using one.

3) The easiest and simplist way is to use a paint marker such as a Gundam marker.

If any paint does stray outside the area, wait till it is dry and remove carefully with a wooden toothpick and Tcut.