Practice! In fact you'll find you get better results with a bigger brush than a smaller one. Get paint on brush start about 1 cm below the transition and move the brush up towards it and along. If you are a bit of a novice do a short distance only effectively "pushing" the paint into the join with the ends of the bristles. Then do it again, and again and again as you move along the wall. Once you get the hang of it you can lengthen the brush stroke into the join.
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