Remove all the furniture you can from the room you want to paint. Shift the larger pieces into the center of the room. Drape a lightweight plastic sheet over the furniture to protect it from any paint spatter. Lay plastic on the floor as well.
Take down any wall decor and set it aside. Remove the nails from the wall. It is important to fill any holes with spackle, as it creates an even surface for the paint. Apply spackle, about a tablespoon amount, onto the spackle knife. Position the blade over the wall at a diagonal and swipe on the spackle with secure strokes. Let dry.
Rub the spackled area with sandpaper in a circular motion to remove any bumps. Clean the surface of the walls with a mild soap and damp washcloth. This will remove surface dust and dirt. Use a heavier cleanser, such as trisodium phosphate, to remove grime from kitchen or bathroom walls.
Apply masking tape to all edges of the room. This includes any door frames, windows and crown moldings. The tape will protect the wood surfaces from paint.
Apply a coat of primer if you are painting over a color that is much darker than the one you are painting with. Primer will also help seal any old odors and stains left over from previous bad maintenance. Allow the primer to dry.
Pour some paint into a smaller container. Dip the 2 and 1/2-inch brush into the container and only allow the first third of the brush to be soaked with paint. Tap against the side of the container to remove excess paint. Make broad, clean strokes right by the areas you protected with tape. This prevents the paint roller from bumping against these surfaces later.
Dip the paint roller into a paint pan and roll it in the paint. Gently roll out any excess. Create an "M" or "W" shape on the surface of the wall. Paint vertical strokes across the shape to smooth the paint color. Allow some overlap to the edging strokes to blend the surface. Repeat until the whole room is finished.
Allow the walls to completely dry, preferably overnight. Inspect your work for any spots you may have missed and apply any touch-ups as needed. A second coat should not be necessary if you applied the precautions during preparation.