Buy a roll of reflective tape. Many sizes and styles of this product are available. Find something that is durable, brightly colored and made for commercial use.
Cut strips of tape that are equal in length to the width of the stairs.
Apply a strip of tape to the vertical edge of each stair tread. If the tape has an adhesive backing, apply it using the adhesive and staple it on for added security.
Apply another strip to the leading edge of the horizontal step, next to the vertical strip of tape, for added visibility. Since people will be walking on this horizontal strip, it should have a finish that isn't shiny or slippery. Add to the safety of the step by applying an adhesive abrasive strip next to the safety strip to improve the anti-slip properties of the step.