Line up one rack mount bar on the left side of the opening you plan to use as your rack space. This space must be 19" across or standard-size rack equipment will not fit. Note that one side of the bar will have multiple holes for attaching gear while the other side will have only three holes drilled for attaching the mount.
Use a drill to install screws in the three holes provided in the rack mount. This will secure the mount to the side of your desk opening or rack case.
Repeat Steps 1 and 2 for the rack mount on the right side of the desk opening or rack case. You can now install rack gear between the two rack mounts.