i do some jobs this way, specially small text. cut a mask, apply, roll on the paint, let the surface get dry, then remove mask. this way, the edge tends to blend better with the background and not be a sharp edge when you run you hand over it AFTER it is totally dry. i still prefer oil base paints for this as they flow better and take a little longer to dry.
if its a job thats 3" or bigger letters, if its a specific font, then pounce pattern like jill said. if its not a tight, strict font, i just kinda layout the letters with a china marker.......till it looks right and throw paint at it))))))) this is the most fun.