So, I got it done, but it took me at least half a day to figure it all out. I was extra careful because I didn't want to ruin the operation of the roof nor lose any parts. I got some nice scratches on my back (I'm 6'3" tall) even though I put some pillows in the trunk. haha
A few tips that I learned by doing:
When detaching the belts just above the rear window while you're in the trunk, unscrew one screw, then attach half of the additional new belt part, then unscrew the other screw and attach the other side of the new strap. That way, the belts don't get carried away. haha
I attached the new straps with two screws instead of just one which meant I had to drill a hole in the nylon strip that attaches the inside lining to the main bow. The good thing is that it really makes that nylon strip super solid now.
One possible issue:
It's not clear to me if the new straps go under or over the attachment point above the rear window. The instructions are confusing and the picture doesn't make it clear to me. It's possible I attached the straps on the wrong side, but the roof works perfectly, so maybe it doesn't really matter.
Another issue:
Now the roof closes very gently, although it still doesn't lock into place when fully retracted. It seems like it's worse now that the straps are installed. I don't think it should make it worse since the straps would be relaxed in the retracted position. I know there are some other threads about how to fi that, but I haven't had any luck on that part yet. So for now, I have to get out of the car and press down on the retracted roof so it locks into place. I'll start a new thread on this issue and see if we can get it handled.
One last thing:
The interior lining has some kind of waterproof material attached to the lining. The picture in one of the above posts shows it. In that picture, it's completely intact. On my car, it's peeling off and I'm not sure what I can do about that. That'll be yet another post! haha