Well, first you need a P2P client using the Gnutella Network (i.e. LW, FW, Phex, Bearshare, etc.).
Then you put the files into your Shared folder. That will make your program gather the hash-tree information of the files (needed by the programs to identify the parts they need to download).
And as soon you go online with the P2P client, your files will be available.
