Paste a length of good-quality painting tape along the line where the walls meet the ceiling. To make it easier to get a straight tape line, use several short sections of tape.
Smooth out the painting tape with a putty knife so it sticks closely to the wall or ceiling surface.
Paint the ceiling, using a brush at the edges so you don't get any paint past the tape.
Remove the painting tape before the paint dries. This prevents the tape from pulling the paint off the wall as you remove it.
Chose a good paintbrush that will allow you to easily control where the paint goes. For best results, find one with a short handle and angled bristles.
Hold the paintbrush like you would hold a pen with your dominant hand. Dip the bristles about 1 inch into the paint.
Turn the brush so it is parallel to the wall. Press the bristles to the edge of the ceiling and drag the brush along the edge of the ceiling, creating a clean line.
Hold the brush so its flat edge is against the ceiling. Go over the line you made at the edge of the ceiling to feather out the paint and allow it to blend with the rest of the ceiling.
Purchase a painting edging tool, which has a 1- to 2-inch pad for cutting in along edges.
Dip the pad of the edging tool in paint.
Press the pad against the edge of the ceiling and drag it along the edge gently.