Go to Hadarac Desert after Eragon talks about Brom and you fight off the two enemies. From the starting point by Saphira, run northeast until you come across a big rock. In front of it there'll be an Eragon-sized rock, a small rock, and some grass. Move foward so that Eragon is facing the front side of the rocks, then jump. If done correctly, Eragon will either float in mid-air or just float down slowly. If you double-jump, Eragon might go inside the big rock. Once done, you cannot get back out and must quit the game or turn off the power to exit from it.