Instructions
Sign up for one of the forums that allow you to exchange game-shared items. GTPlanet is one of these forums.
Check the format that you are using to upload and download game-shared items. NTSC is the North American format; if you are looking at PAL format files, those will only work in Europe.
Download WinRAR Archiver if you don't have it already, because most game-saves and other items come on compressed files that you need to extract with that sort of application.
Upload your game-shared items to your forum in a post. Some forums have an attachment feature that lets you send the file directly. There is a limit on file size, so keep your file size beneath that limit or compress the file first.
Stay away from game-shared items of the SharkPort, CodeBreaker, Max Drive or X-Port type. These are not compatible with your PS3's virtual memory card.