The correct answer is D) Lay sandpaper between the brush and the slip ring and slide the sandpaper back and forth under the brush. This is the first step in seating new brushes on slip rings. The sandpaper helps to conform the brush surface to the shape of the slip ring, ensuring a proper fit and contact. This process is necessary to ensure good electrical conductivity and minimize brush wear. The other options are incorrect because they represent later steps in the brush seating process, such as applying pressure (A and C) or using a seating compound (B), which should only be done after the initial sandpaper seating.