Cover the entire floor with painter's tarp. Covering the entire floor, rather than moving a tarp as you go, saves time and offers better protection once you begin to paint.
Develop a plan for painting the room. Work on one wall at a time. Decide if you want to work from the left side of the wall towards the right side or vice versa. Don't start in the center; choose the left or the right side.
Cut in the edges with a paint brush. Start at the top of the wall and cut in the edge between the ceiling and the wall. The cut in line should be about 2 inches. This makes it easy to paint the wall with a roller without bumping the roller into ceiling. Cut in the corner of the wall as well. Avoid getting paint on the adjacent wall. The idea is to keep a wet edge only on the wall that you are working working on. Finally, cut in the edge along the baseboard trim and the bottom of the wall.
Attach the paint roller to an extension pole. Painting the wall with an extension pole is much faster, which makes it easier to paint a wet edge before it has time to dry.
Paint the wall with the paint roller working in the same direction used for the cutting-in process. If you cut in from the left to the right side of the wall, begin rolling on the left side, which has had more time to dry than the right.
Saturate the roller with paint in a paint tray. Work from the ceiling towards the floor with the roller. Roll it up and down several times, covering a 3- to 4-foot wide area. In addition to maintaining the wet edge from the cut in line, it is also important to maintain the wet edge with the roller line. Overlap the wet edge of the first section when you roll the second section of the wall. Continue the procedure until the wall is completely painted.