Start by putting painter's tape on the area that you are working on. The tape will allow you to mark the area as needed and will also help keep your paint from being scratched. Also, be sure you have selected an area with room for a drill to be used. Determine the minimum size hole needed to pass your item through the firewall. (What is the largest piece that must go through?) Make sure it is smaller than the outside dimensions of the grommet (1.25" is the maximum hole size recommended). When drilling is complete, you can remove the remaining tape. If you are using the grommet for a harness, we suggest that you put the largest connectors through the hole first and work down to the smallest. Note that the black junction cover must be removed to fit it through the hole. Once the harness is through the firewall, you can install the first piece on the front and the jam nut on the back. Now center the grommet and ensure it is covering the hole cleanly. Finish by installing the second piece of the front plate using the two small Allen head bolts.