Assuming that your rear dropouts are horizontal. (if not never mind) Loosely assemble the dropout spacers/stops and slide them into the dropouts, big end towards the back. Install the wheel and make sure that its straight and not cocked to one side. Slide the spacer/stops against the axle and tighten the screws on the drop out spacers/stops. Otherwise you will be cursing yourself every time you remove and reinstall your rear wheel as you try to get the wheel straight in the frame. Also if you don't install them, your axle will slide in the dropouts and eventually the rim will rub the frame and/or brakes.