The easiest way to cut in is to get an angled sash brush. Then dip the brush wipe the excess paint off or shake the brush in the can. Wipe some of the excess paint on the wall. Then use the tip of the brush. Either push or pull the brush using the tip to aim. Steady hands help too.
Try as I may when painting I always get it where I shouldn't which normally means reprinting where I have already painted how do I cut in properly
