Select the correct size grommet for the opening and the diameter of wire that will be placed through it. If the opening the grommet will be placed into has not been made yet, consult the grommet guidelines for recommendations about the opening size. All grommets will have an internal “panel” hole diameter and an internal hole diameter for the wire(s) to pass through.
Drill the required hole/opening for the grommet, unless this is a pre drilled hole then ignore this step. (Remember to de-burr around the hole, this will aid the fitting of the grommet).
Push one side of one rubber grommet edge into the opening to begin inserting the grommet, it may be necessary to use fairy liquid or similar to help the grommet slide in.
Manipulate around all of the edges to insert the grommet. It is okay to bend the rubber grommet as necessary when working the grommet into the hole, it is easier using both thumbs if possible to push the grommet, once in you can press around the internal hole of the grommet to seat it properly.
Insert whatever wiring or tubing you are placing through the grommet through the hole of the grommet, and your done.
I hope that this helps, the smaller grommets can be more difficult as the hole is very small, although not ideal, if you do have difficulties you can cut the grommet so that it is easier to install, if done carefully when finished you would not see the cut, however this is only recommended if all else fails.