I just got my 818 retrofit hard top 2 days ago but I thought I'd start a thread where we can list some installation tips.
Headliner:I learned that 3M Headliner and Fabric adhesive ($18.99) doesn't work holding our aluminum headliner to fiber glass. I followed the instructions exactly with the hard top upside-down and weighted for 4 hours. On the car overnight in the garage it came down. Next, I'll try silicone sealant and report back. I'm in a perfect test location with 115 degrees today!
Hatch hinge: The manual (1P) says to mount the hinge arm to the hatch with 1/4x20 bolts. What it doesn't say is that you should first mount rivnuts in the hatch cover. Nate at FFR said that there is aluminum inside the molded hatch cover so rivnuts will hold.