You upload them to the FTP.
Read
here on how to go about doing it.
In short:
Get a FTP client (
FileZilla seems to be the choice around here, and it is VERY easy to use), log into it, make a folder, and upload the file in that folder.
Protip:
For super-long Longplays, do this:
Transfer -> Default File Exists Action
Set "Upload" to Resume.
Trust me. It is NOT fun to keep renaming the same file because the upload stopped due to connection crapping out or otherwise.