Mel, just some Blue painters tape rubbed down well on the inside (stripe) edges ... Easy !!! If you define the edge with a chalk line first , it helps laying out the tape straight ... Just fixing one end and stretching the painter’s tape free hand and eyeball is no guaranty of a straight line

... Tried that for years , finally gave in to actually marking where the tape goes ... Much better job now ...
JMHO, a thin red stripe livens up most any paint job !!!

Not to be overdone , just a little accent...
