A good sized Root Top Tent (RTT) is awkward to manoeuvre and can be heavy to lift. The following shows a sequence I use to secure an ARB Simpson III RTT on a Toyota 4Runner.
Protect the vehicle? I used moving blankets.
Locate a suitable platform to rest the RTT before lifting.
Position the RTT on the platform and resting against the vehicle, protected by the blankets.
Use the roof rack as a pivot point to raise and push the RTT in to position.
Position the RTT in the middle of the rack.
Consider what other items your rack will carry before finalizing the location of the RTT on the rack and then remove the blanket.
I used blocking to raise the RTT so that the rails were exposed and accessible for the fixing hardware.
Secure the RTT with the fixing hardware to the roof rack.
Good to go!