Draw a line for the border with a pencil and a straight edge.
Dip a two-inch angled brush into the paint and try a few practice strokes on the side of the border where you want the paint. Insert the bristles one-quarter of the way into the paint. Find a comfortable angle and apply a paint stroke parallel to the border line. As you draw the brush, you will notice the bristles aligning (if you are using a good brush) and creating a perfect edge. It will stay straight if you pull the brush straight.
Load the brush when ready to paint the edge of the border. Start a stroke just away from the border line and approach it as you pull your stroke across. Continue to pull the brush across, even with the pencil line, until the paint begins to run dry or you feel you cannot keep the line straight. When that happens, slide the brush away from the line and lift it off the wall.
Reload the brush and repeat. Continue until you have completed the border.
Use a damp rag to quickly wipe any paint the goes over the line.