THIS IS NOT MY TUTORIAL! All credit goes to Berion over at PSX Scene.
If You are CFW user i recommend a Rebug CFW which brings nice debug fe called Fake Save Data Owner. It's turned on by default on all versions, which saves you a lot of inconvenience, and this mini-turorial will be a needless if You are on this CFW. If You are on other CFW or OFW this tutorial is for You. ColdReader (PS3SaveUnlocker) write a nice app which automate resigning process in PARAM.SFO but in my opinion hex editor is sufficient for such an easy task (and PS3 Save Unlocker unnecessary copy whole data block from first id to EOF) but of course, first you should try his program. If game still don't recognize save do this as i described below:
1. Open PARAM.SFO in any hex editor (i.e Hex Workshop). It doesn't matter which hex editor because all can represent data in raw and have copy/paste/find options which are sufficient. You need replace (very important: not add/del or something, just replace) a few things:
2. First is the "user id" which identifies Your user account. This ID always start on the same offset (0x140) and always have 16 bytes. Replace it by Your values - and do this twice - first on 0x140 and second somewhere in the file (address of 2nd "user id" is constant only for the same game save title, it's different for different games. Use search option in hex editor).
Note: Most saves downloaded from internet has zeroed this area and it's difficult to find proper 2nd offset. Don't worry, in this case just compare to files.
3. Second id is "console id" which identifies Your console (don't mislead it with IDPS). It also have 16 bytes and also is not on constant offest. For example, in Flow (NPUA-80001) is on 0x574.
4. Third id is user number account (for example: for "dev_hdd0/home/00000001/" will be 01h)). It's has 1 byte and be situated on next byte after "console id". For example, in Flow (NPUA-80001) is on 0x575.
That's all. If You all did correct, game will recognize save file and not locked save option and trophy system. If not, this mean that this game save is PARAM.PFD protected and we cannot do anything until someone wise reverse this sh*t.