In order to save bandwidth the Final Fantasy Tactics: Rebirth mod has been re-released as a PPF mod. In order to use this version of the modification you will need two things. An ISO/IMG/BIN of the original Final Fantasy Tactics and an emulator. You can rip your own Tactics disc to create the ISO/IMG/BIN or download one from the internet if you have the PSN version of FF Tactics. I would suggest visiting Fantasy Anime for assistance in downloading a FF Tactics ISO/IMG/BIN if you need one - Fantasyanime.com
You will also need an emulator to play the modification. If you are not familiar with emulation I suggest you check out this great guide from Fantasy Anime: Fantasyanime.com
I recommend downloading ePSXe from Next-Gen Emulation. Here is a link to their emulation section: Forums.ngemu.com
You will need a BIOS, GFX Plugin, and Sound Plugin also. Remember to use the Fantasy Anime guide if possible, it will fill you in with the basics.
After you have acquired all those things please head over to Moddb.com and download the modification.
Use the following tutorial video to install the mod (instructions are also included in the ZIP file in a Readme document).