Border stencil (square, purchased from Michaels stores, cut in half)
What to Do
Decide on the height of your stenciled border.
Base-coat the walls from the ceiling to slightly below the decided height with lighter green; let dry.
Using a level and color pencil, make a horizontal line around the room at the desired height.
Tape off the upper portion along the drawn line with painter's tape. Base-coat the walls below the tape with darker green, using a clean roller. Remove the tape; let dry.
Spray stencil adhesive onto the back of the border stencil and position it along the paint line. Use the stencil brush to stencil with the lighter green paint. Carefully remove the stencil and reposition as needed to complete the border. Let the paint dry.