After you make the back up, reboot. Once booted up, download flash GUI app if you haven't already.
Open flash GUI select the rom.zip, let the app load it to a temp directory (it'll have a dialog saying 30-45 seconds) it may take a little longer, just be patient. Once it is done doing that, hit the flash button at the bottom of screen. After it is done flashing it, select the middle option of the pop up > "reboot to recovery"
Once in recovery select "Wipe" then factory reset, once that is done DO NOT REBOOT, back out to main screen of TWRP, select "Install" then select viper 3.0, and start the install.... Viper uses Aroma installer, which is excellent... When going through the installer select Wipe even though you already did this in TWRP, and select the default stuff; North America, etc... Don't delete any sense stuff and it should be all good... First boot will take some time, don't worry, it'll boot... If not just do these steps over again.