Place a ladder stabilizer on the top end of an extension ladder. Put both feet of the ladder on the tread of one of the lower steps of the stairway, with both feet against the step's riser. Lean the top of the ladder with the ladder stabilizer in place against the wall opposite the top of the stairs.
Place a stepladder at the top of the stairs. Make certain the feet are level and secure.
Put the 2-by-12-inch board on matching rungs of the two ladders to make a level support to walk on as a scaffold. The scaffold board must be long enough to span the distance between the ladders, with some overlap at each ladder.
Secure the scaffold board to the stepladder at the top of the stairs with a C-clamp.