It’s called “cutting in”—or just “cutting”—and you’ll take care of it with your 2½-inch paintbrush. Before you lift a roller, cut all the corners in the room. Side note: Whenever you’re working with a brush, dip the bristles only about a third of the way into the paint, then gently slap both sides of the brush against the inside of the bucket to remove excess. (Don’t scrape the brush on the bucket’s edge.) This will distribute the paint more evenly. Here’s how to cut.
1. Saturate the corner and stroke outward. Press the tip of the brush to the wall, shimmying it into the corner for full coverage. (It’s fine if paint gets on the adjacent wall—you’re going to paint that spot next.) Then brush outward a few inches. Create a stack of about five overlapping strokes. Redip the brush as you go, if needed.
2. Smooth and eliminate brushstrokes. Dip the brush, then run it over the short strokes in a long, perpendicular stroke to even out the coverage. Later on, the roller will smooth any remaining jagged edges. When you’ve finished one section, repeat the process on the adjacent surface, gradually working your way around the corners of the room.
For more tips, see How to Paint a Room.