Cover the floor, counter, toilet and other surfaces with plastic tarps.
Tape off the edges of mirrors, ceiling trim, windows and other areas that you want to protect from paint with low-tack painter's tape.
Remove any outlet and switch covers by unscrewing the screws that hold them to the walls and set them aside.
Open any windows and turn on the fan for ventilation.
Scrape off any loose paint with a paint scraper. Wear gloves and a respirator if the paint was applied before 1978 as it may contain lead.
Sand existing gloss or semi-gloss paint with 120-grit sandpaper and a sanding block until the shiny surface has been dulled.
Wipe the walls with a damp rag to remove the sanding dust.
Open a can of latex primer and pour some into a paint pan.
Dip a paint roller into the pan, moving it back and forth to coat the roller completely.
Apply a coat of primer to the wall, starting near the bottom and making large zig-zag strokes. Do not paint all the way to any painters tape.
Paint edges near taped areas using a small roller or brush. You will have more control with these tools than with a large roller.
Allow the primer to dry for 24 hours.
Apply a coat of latex paint designed for bathroom use, following the same procedure used for the primer. This paint is made to be more resistant to water and steam damage than standard latex paint.
Allow the paint to dry, then apply a second coat.
Allow the paint to dry overnight.
Run a razor blade along the edge of the painters tape to prevent paint from adhering when you peel it off.
Remove the tape and tarps, and replace any covers to the walls.