I am going to assume the rear rack has been installed on your bike per the manufacturer's directions.
To attach the rear runner piece, lay it on top of the rear rack and mark four spots for holes in the cross pieces. These holes should be near a tube or edge of the rear rack. The wheels of the runners need to be facing out. The other end should not intrude on the seat area.
Drill the four holes. (Size them appropriately to your stock and your zip ties).
Zip tie the runner to the rear rack.
Note: I had experimented with other ways to connect the runner to the rack - I wanted easy on, easy off, and as it turned out, the zip ties worked best. You may work out something else. I'd love to know what it is.