Allow the walls to dry for at least 48 hours. Repainting when the walls are still damp will make the new paint dry unevenly.
Apply painter's tape along the ceiling above the walls you'll be painting. Painter's tape will keep fresh paint from getting on the ceiling and won't damage the ceiling's finish.
Pour primer into a paint tray and apply to the walls using a paint roller. Primer will help the new paint stick to the old paint, and will help cover particularly bold or dark colors. Allow the paint to dry.
Pour interior paint in a color of your choice into a paint tray. Apply to the walls and let dry, then apply a second coat. Let this coat dry for at least 24 hours.
Remove the tape from the ceiling. Touch up any spots along the top of the wall that look uneven, using a cutting in bristle brush to apply the paint. Touch up any other spots that look like they need a bit more coverage using a bristle brush.