To unlock the Fly, You Fools! trophy in Far Cry New Dawn, you will first need to play through the story until you unlock the Carmina boat/wingless plane at the end of The Great Escape mission.
Spawn the Carmina at the dock in the far north on the map, and then carefully drive it to towards a cliff with water below. Get a bit up a run-up and then drive off the edge and pull back on the left stick (L3) whilst in the air. This should easily total more than the 100m required to unlock the trophy.
Check Maka's video below for the exact location if you're struggling to find a good spot.