Video guide available here:
First, download the .xdelta file for the game version you want to patch, either PSP or PS3. You can find a link to the patch on the main page, under the "xdelta" section.
Then, if you haven't already, install xdelta using your package manager.
now run the command 'xdelta -d -s "original_file" "xdelta_file" "output_file"' where you replace the text between "" with paths to the corresponding files: original_file with the path to the japanese game .iso, xdelta_file with the path to the downloaded .xdelta file, and lastly replacing output_file with the location where you want to save the english patched .iso file.
Here's an example of how that might look like:
xdelta -d -s ~/Desktop/game.iso ~/Desktop/psp_patch.xdelta ~/Desktop/output.iso
Now, hit enter and wait. Once the program exits, you should find the output iso in the location you specified in the previous step.
You can open this .iso in your PSP emulator.
Keep in mind the PS3 version also has to be run in a PSP emulator.
Of course, if you want to play on a real PSP, you will need to patch the PSP version.