Cover the stairs with a drop cloth. Secure the drop cloth with tape, or place heavy objects at the top and bottom of the stairs to ensure that it does not slip.
Place an extension ladder on a stair about halfway up the stairs. Lean the ladder against the wall. Ensure that the ladder fits snugly between the stair and the wall and all safety locks are engaged.
Open a 6-foot step ladder at the top of the stairs. Place the ladder at least 8 to 10 inches from the edge of the stair for safety. Lay a 2-by-8-foot board, or a walkboard, through the first or second rung of the step ladder, across the stairwell and through the rungs on the other ladder, creating a level surface to stand and walk on over the stairwell.
Use a C-clamp to secure the board to the step ladder to prevent the board from shifting.
Stand on the walkboard, and paint the cut-in work with a brush. Paint the remainder of the stairwell walls, using a roller attached to an extender for safety and comfort; an adjustable extender can reach up to 12 feet, allowing you to safely paint the upper stairwell walls to the ceiling.