It's really easy, first you'll have to check "dump textures", preferably before loading the game.
Then you should disable "EFB Copy" otherwise, each EFB frame will be saved as a new texture which is of no use.
Then every loaded textures will be saved under User/Dump/Textures.
Once the textures are dumped, you can modify them and put then under User/Load/Textures. Then they'll be automatically loaded in place of the real ones if "Load Hires textures" is checked
It's really as simple as that!
HumHum :
