Before you download the tool, all the PS3 saves on this website are already resigned to work with RPCS3. Only use this if your trying to use saves for some other game.
Save folders for PS3 are in BLUS, BLES, or BLJP format.
BL means Blueray, while US / ES / JP are the region versions. US is usa, ES is europe and JP is japanese.
Save files only work with a game of the same region, so make sure your save your trying to uses is for your game version.
Anyway, to use this thing to make saves work with RPCS3, after installing save resigner, open it. Drag and drop your BLXX folder (the folder itself) into the save resigner. In the popup window, click decrypt, and thats it (it saves automatically). You can close save resigned and use your saves.
The default PS3 Save Resigner Folder is installed to:
"My Documents/The Prince of Codes/[PS3] Save Resigner"
Some games require more then just resigning a save for RPCS3 to make them useful. Some game require you to do something ingame as well. For example, in Tales of Vesperia PS3, having a clear save makes the new game plus menu appear, but you can't actually use it without crashing unless you load that save and go kill the final boss, to write some system flags to your PS3 settings. Similarly, Tales of Xillia 1 does a similar trick, but even more ridiculous. In Xillia, you have to legitimately play the game untill you recruit alvin and see the anime opening, then save the game to set a flag in system settings that unlocks the TITLE SCREEN. otherwise, you can't use your saves. Other games may have other ridiculous requirements.