Move furniture, if there is any, to the center of the room. Cover the furniture with plastic tarp or old sheets to keep from getting paint on it. Throw a tarp over the floors, as well, to make sure the floors don't get paint on them. Line old newspapers around the baseboards of the entire floor to catch any paint splatters. Tape the newspapers down on the floor to prevent them from shifting.
Dust and vacuum the room well. Nothing ruins a paint job like dust sticking to wet paint. Apply strips of painter's tape to the baseboards so that paint does not drip on top of them. Smooth the tape down firmly as you align it, taking care to pull the tape in a straight line; the line you make with the paint tape will be the paint line you leave on the wall. Run a strip of painter's tape along the ceiling line as well. Put the tape either on the ceiling near the crease of the wall or use a very thin painter's tape to line the top of the walls.
Remove all face plates from electrical outlets and switches throughout the room. Unscrew the face plates and tape the screws onto the back of each plate for easy access when putting them back on. One hallmark of a well-painted room is face plates that have not been painted on or over.
Remove the curtains, if possible. Lift the curtain rods up and move them to another room, with the curtains attached to the rods, to make it easier to put them back. Pull blinds up all the way and wrap the dangling cords into the blind slats to keep them from getting painted on. If painting inside the window frame, it may be necessary to entirely remove the blinds. You can paint around the blinds inside the window frame, if desired, because it makes the job easier.
Begin painting the room by first painting along the top of the room, circling it completely. Paint along the bottom of the room, also circling the room. Because you have already taped off areas, paint should not get onto the ceiling or baseboards. Use a paint roller on large walls or a paintbrush on small walls to apply the paint. Use broad overlapping strokes to insure even coverage. Use specialty paintbrushes in the corners, if desired.
Remove the paint tape just before the paint is entirely dry, or it might pull the new paint off the wall. Examine the final paint job with good lighting to catch areas that were missed. Use a small craft paintbrush to touch up small areas.