I'm not saying my way is the best way, but it has worked well for me. I put down a bed of 3M Marine Adhesive Sealant Fast Cure 5200 under the mounts. I then put two stainless steel screws through each mount into pre-drilled holes. The last step was to cover the mounts with Dycor.
The panels have been on the roof for about two and a half years and are holding nicely.
2016 Sunseeker 2300C
"The whole problem with the world is that fools and fanatics are always so certain of themselves, and wiser people so full of doubts." - Bertrand Russell