I did not create the video below; all credit for that goes to Xboxlivefr on YouTube.
If you're doing an achievement clean-up after completing the game, this achievement can be started in roughly the same area as The Longest Glide ends.
The The Longest Glide achievement in Fe worth 295 pointsGlide from the top of the waterfall all the way to the marsh
Note: To land on the back of an owl, you must first speak to it to get it to trust you. Therefore, to get this achievement, you need to know how to speak Bird (your second language), as well as how to climb trees and how to glide (your first and second abilities).
The video below helped me net the achievement, though I feel it makes things too complicated. Begin by approaching the first owl as seen in this video, which can be found on the map above the blue circle of water. There are a few birds in this area; make sure you choose the right one.
After you speak to this bird and get it to trust you, hop on its back and let it take you in the air. You'll see a few other owls flying around, and you'll fly over one that lands on its nest. Hop off the back of your owl and land on the ground near it. Speak to this owl, and it will trust you.
Now, return to the spot of the first owl. If the owl is not there, hit
to call it to its nest. Hop on its back and it will follow the same path as before. Now, instead of landing next to the second owl, jump and glide and land on its back. Success!