The first step to take is to purchase a rail guide assembly that is the perfect fit for your bread box. If you can't find one that fits your box exactly, get one that's slightly larger and then cut it to size.
You want your sliding shelf to match your bread box so pick a matching wood and color. The shelf should be no thicker than 1/2 inch and should just barely fit between the rail guides once they're installed. Leave a gap no larger than 1/16 inch.
Use the hardware that came with the rail guide to attach it to the inside of the bread box. Mount the rails uniformly on either side and ensure they're perfectly horizontal. Now attach the rollers that came with the rail guide to the back and front corners of the shelf, then slide the rollers into the rail guide.