Protect the floor under the wall you are painting by spreading drop cloths over it. Ensure that the cloth reaches all the way up to the baseboard. Remove all hardware and outlet covers from the wall using a screwdriver and set them aside.
Apply strips of painter’s tape to all trim and molding that you do not want to paint, making sure that no part of them remains exposed.
Look at the wall and identify where to change paint colors. To create a horizontal division, measure up from the floor at the desired height and draw a pencil line. Repeat at 3- to 4-foot intervals along the wall and stretch a chalk line across all the marks. Place a level on top of the chalk line and move the line up or down until it is level. Snap the chalk line to create the division mark. To create a vertical division, hold the string end of a plumb line at the top of the wall. Wait until the line is still and place a chalk line next to it, snapping it to create the vertical division.
Apply long strips of painter’s tape to the wall surface on top of the chalk lines. Press the tape down as hard as you can along the edges.
Open the first paint color and pour it into a paint tray. Insert a paint roller into the tray and roll the paint over one section of the wall, being careful not to go over the painter’s tape that marks the division point. Wait for the paint to dry and then apply a second coat.
Let the paint dry overnight until it is no longer sticky. Remove the painter’s tape from the division mark and throw it away. Place a new strip of painter’s tape on the painted portion lining it up perfectly with the edge.
Open the second paint color and pour it into a clean paint tray. Insert a fresh roller into the second color and apply it to the unpainted portion of the wall in the same manner. Wait until it dries and apply a second coat of paint.
Wait until the paint dries and peel off all of the painter’s tape from the wall and molding. Roll up the drop cloth and throw it away. Reinstall all hardware and electrical covers on the wall.