Complete your first Forza Motorsport movie.
Finish any single race you like, then as the menu pops up, select View Replay. Then select Make Movie, which is the third icon from the right during the replay, set quality as anything you like, but keep Movie Length as 5.0 s, and keep game speed at 50%. Then select create movie with and it will start to upload. Once the upload is complete, you will get the achievement.