Using a large cellulose sponge and a solution of water mixed with a few drops of mild dishwashing liquid, clean your walls to remove any dust, dirt, and grease.
Use blue painter’s tape (not masking tape) to protect any areas you don’t want your paint to get on, such as trim, molding, doorknobs, window frames, and door frames: Run long strips of the tape just inside the outer edges of these areas. (The outer edges of the tape should lie exactly where the wall meets the trim, covering the parts of the trim that your roller or paintbrush might hit when you paint.)
Pour in enough so that it almost fills the well toward the bottom of the tray without covering the angled portion of the tray where the ridges are.
Dip your roller into the primer, rolling it back and forth across the tray’s ridges a few times to remove any excess and prevent drips. Make sure the roller gets evenly coated. Run the roller up and down a section of the wall, applying primer until the section is fully covered. Continue until your wall is fully covered, reapplying primer to the roller as needed. Tip: Follow the manufacturer’s drying instructions, which you’ll find on the can, to make sure the primer is completely dry before applying paint.
Dip a two-inch angled brush into the paint, coating the bristles only about a third of the way down the brush. Run the brush along the outer edge of the tape that’s covering the trim (on the wall side of the tape; not the trim side). Keep applying paint flush against your taped trim, working it outward about two or three inches from the tape. Continue until you have finished painting a narrow swathe along all of the taped areas.
Fill a paint tray with paint; dip your roller into it, removing excess. Roll the paint onto the wall in a 3-by-3-foot “w” shape. Working your way back across that “w,” without lifting the roller, fill in empty patches until that section of wall is fully covered. Continue, adding more paint to the roller as needed, until the whole wall is painted.
Peel off the tape while the paint is still wet to avoid accidentally removing any dried paint along with it.