Practice.... It's a skill. Wish I was a wordsmith then I could write how to do it instead of doing it all day in and out....
Not being funny BTW but it is somtheing you need to practice. Try on some plasterboard offcuts.
Use a 4" brush for cutting in walls, half load brush, strike a line a fraction away from the finish edge, (far as u can stretch safely) then without reloading brush go over the same area again this time onto the finish edge.
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