I did not create the video below; all credit for that goes to Rare Ultra on YouTube.
This glide is far, but it's very easy to accomplish. I watched the video once, then completed it on my first try as I played the video again. Note: To accomplish this, you'll need to have gained your third language (Deer, the shout that opens the flowers that create a jet stream), as well as the ability to glide (your second ability).
Begin by getting to the waterfall, which is the light blue X on the map halfway between the purple marker (where you get new abilities) and the light blue circle (which you'll fly over). Climb to the top of the tree (as indicated in the video), set your language to the upper-left most option (Deer) and glide south. Hold
and push slightly forward on your
to glide without losing too much momentum. As you glide, keep speaking (using
) to continually open the jet stream flowers. Follow the path in the video (when in doubt, stick to the water/jet stream flowers) until you reach the end.
There's only one semi-tricky turn in the entire path, and it's near the end (the one near 1:28 in the video). There seems to be quite a bit of lee-way in terms of getting where you need to go, so as long as you're comfortable with the controls of the game, you should have no issue unlocking this achievement.