When you go to put the fairing on swing it out at the bottom allowing that gap to click together.Wiggle it from front to back until that joint is achieved.Don't move on until it does. That join and the one below are always the toughest and have to be done prior to any others.There are 3 slots in there that need to click into place.Don't force them down however tempting.
For those that have scratched fairings [both Gen's] when putting them back on always have a huge supply of painters 2" wide tape on hand.Any edges that come close together during re-assemble should have a protective layer put on. PITA,takes a moment but well worth it.Not to brag or be "the Man" here but I can sling those things off/on pretty qwik.20+years of Busa plastics comming off on a regular basis,both Gen's. They can and should look like new when put back together.
Just ask RedBull to find that video,it may help.I've watched it,but can't swear it will help with this stage,but it is a good noobie thing.
Rubb.