Start with clean, white or off-white walls. If the paint job on your walls is old, you may wish to apply a fresh coat. Move all furniture away from the painting area and lay down newspapers to protect the floor. Press painter's tape around the edges of windows, the floor and ceiling to keep paint from getting on them. Match the paint to the your current wall color and pour it into a painter's tray. Dip a clean roller into the paint, and roll it on the upper edge of the tray to distribute the paint around the roller. Apply the paint to the wall with long, even up and down strokes. Use a small paintbrush to fill in edges and corners. Allow the paint to dry overnight.
Plan your design on graph paper before decorating the wall. Create your design as a "to scale" model on the paper. Measure your wall to accomplish this. For example, if your wall is 20 feet wide and 10 feet tall, then 2 inches on the graph paper equals 1 square foot. So, draw your design on graph paper 40 inches wide and 20 inches tall.
Use painter's tape to create your design on the wall. Cut the tape to create your design. Be creative -- use pinking shears for interesting edges. Use pieces of tape to apply the design or picture to your wall. Create large, curvy designs with multiple pieces of painter's tape. Overlap edges of tape to keep wet paint from leaking through.
Pour colored paint into tray, dip the roller and run it along the upper ledge of the tray to evenly distribute the paint. Paint the wall using long, even strokes. Paint up and down, not side to side. Use a small paintbrush to fill in the edges by windows and corners. Allow the paint to dry overnight.
Remove all painter's tape by pulling it back over itself. Reveal your design.