First, it's very important to note that cross save only works on the first saveslot, the rest don't support cross save function.
The cross save is cloud based, so you don't need to do anything special to have it working, it's fully transparent, apart from the fact
that the first time you save in the 1st slot and are signed into PSN, it will take a bit longer to perform the save (and if you make a xtra large graffiti, you'll notice longer saves too).
Once you have saved your progress at least once in one platform, it will automatically sync to the first slot in the other one. You'll notice a small "sync" icon next to the 1st save slot date and time, indicating that there is new data online that will be synced on load.
The data sync works differently for custom graffitis and save data:
- For graffitis, the newer ones overwrite the older ones, that is, if you create a new graffiti on PS3, and save while online, when loading the game in vita, it will replace the one there.
- For save data, when a platform notices updated save data online, it will download it and combine with the current progress, so the progress is added.