This thing that worries me is the statement: "Requirements: You must have an unlocked bootloader this contains a modified kernel."

OK, I figured it out, see the post below.
It worked!
If you've installed the CASUAL rooted version 4.1.2, it installed the unlocked boot loader (TWRP comes up when you do a Vol Up/Home/Power startup sequence.)
You have to use ODIN to install the Wifi/Modem update.
Once that is done, copy the big recovery zip file to a SD card. Restart you phone with the Vol up/Home/Power buttons. TWRP comes up. Do the factory reset, then use the Install to install the zip file you put on the SD card.