First you must have an xbox live gold subscription. Then press start in any lobby. In the first tab there should be a button saying File Share. Press A on that, and it will take you into your file share. In your file share, press A on any of the empty slots and it will give you the option to upload to your file share. From there you will select the type of file (screenshot, film clip, film, map, game variant), and press A, and then choose the one you want to upload. The upload will take anywhere from 2 seconds to a minute. And then the files and in your file share.