Finally getting to this project.

First, simply open the zipper in the headliner (assuming fabric) and take a look. This view is straight up behind the pilot seat looking at the carry through spar that runs from side to side. Note the factory-installed nut plate in the spar. You will be using that. Be sure to run a 10-32 tap through it with some light oil to clean the threads.
In my case, my comm antenna is forward of the spar and right in the way of where the inertia reel will mount.

This is the bracket that the reel will bolt to. We need to get that mounted using the one provided hole and two that we will make. With the bracket in place, mark its width on the flange of the spar where it is riveted to the outer skin. Then remove the bracket to see which rivet(s) will need to be removed.

Probably not an issue in most installations, but for me, this antenna has to be moved.

Comm antenna on top of fuselage as previously mentioned.

A little tape on this automotive upholstery tool protects the paint while I get the antenna loose.

This is the rivet we determined in a previous step had to be removed from the spar to allow the inertia reel bracket to mount flush.

Like any drilling job, it's important to make an accurate punch.

I started very small and drilled in three steps to get to 3/16"

I decided it would be smart to give the paint a little protection from any sloppy drilling.

Then I decided to make a chip catcher to stop the mess from running down the plane.

It worked well.

Then I temporarily installed the bracket again to mark the first hole.

And then drill it in the vise.

Also to 3/16"

Measure and mark for the second hole. You get to choose where to drill the second hole.

Where to drill the second hole is just based on logic. The bracket needs a second hole. Where should it be?? Then drill it.

Once again with the chip catcher. When its full, just fold it in on itself and throw it away.

Temporary install of bracket again gives you the seconds spot to drill.

Now you can bolt the reel to the bracket.

My tip here is to pull all of the belt out of the reel to make room to get the screw driver in.

Reel and bracket bolted in. You can see through the old antenna hole that there is no room for that antenna here.
My tip here: I screwed up. I got all done and then made my patch panel. I found out that there is no way I'm going to be able to attach the patch plate with the seat belt reel in place. So...... I will have to pull the reel back out (but not today).

From the inside.

Here, I went rogue a little and ignored the instructions. The trim plate mounts to the single bottom screw that mounts the reel bracket to the spar and the seat belt guide just fits through it. The instructions say to locate this screw with the head liner zipped closed. Measure to here and mark; measure to there and mark; then to more marks. Now cut the fabric based on your measured marks. The hole they have you make is substantially larger than the opening in the trim plate that the seatbelt passes through. I decided to remove the screw from the spar, zip the headliner closed, find the screw hole through the fabric and bolt the trim plate in place. Then mark the opening that needs cut, remove the trim plate, cut, and reassemble. This method seemed less risky to me.

Bracket removed for cutting.

Finished product. I don't suspect I need to cover bolting them to the floor.