Buy a PC code for FIFA 16 online, then redeem it with Origin, and download the game.
Then, when it appears to have downloaded, start the game and ensure that it has (sometimes it will say that it is not fully downloaded). If it hasn't, go into Origin, go to FIFA 16 and click "check for update". This should start the remaining download.
Once it has fully downloaded, and you've started the game to confirm it, quit the game and rename the FIFA 16 folder to e.g. "FIFA 16 Origin Version".
Then, download FIFA 16 v1 from the FIFA 16 thread and unzip it to the same place where the "FIFA 16 Origin Folder" is - NOT overwriting it but alongside it, calling it "FIFA 16" or whatever you originally called the folder when Origin installed the game.
(By alongside it, I mean - say the Origin version was installed to
C:\Games\FIFA 16 Origin Version, once you've renamed it, then extract v1 to
C:\Games\FIFA 16, which shouldn't currently exist.)
Then, to be safe, delete the FIFA 16 folder from your My Documents folder.
Then, start Origin - put it in offline mode - and try to boot FIFA from the desktop shortcut (which should launch the v1 FIFA EXE).
It will ask you to confirm your Origin email address and password. Once you have done that, it should never ask you again.
If it works - GREAT! Then, quit, and (if you're going to install patches), delete the My Documents -> FIFA 16 folder again.
Then, you install any patches and mods you like to the
v1 FIFA 16 folder as you normally would. That folder being C:\Games\FIFA 16 now.
Follow the instructions for those mods, which will usually require "regenerating" afterwards (see below).
That's it!
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You can just drop in other people's cl.ini files (which let you change most attributes) into the folder, to change gameplay massively.
The most difficult bit is "rebuilding" after installing each mod - you need to download a tool that basically builds an index file for the game to "read" (like Creation Master 16,
download here) so all your new files get picked up and used by the game. But all you need to do for that is install the tool, point it to your FIFA 16 directory after you've unzipped all the mods there, and click "rebuild".