Go to the XDA forum for Team Venom's ViperXL 3.2, scroll till you see the download link.. download it (it will be located in "Downloads" folder).
BEFORE FLASHING ANYTHING, IT IS ALWAYS SUGGESTED THAT YOU MAKE A BACKUP OF YOUR CURRENT ROM/STOCK (although you can always find the stock Ruu on htc's website or on the forums)
FOR VIPER XL, YOU DO NOT HAVE TO FLASH BOOT.IMG, BUT IT DOESN'T HURT TO DO IT "JUST IN CASE"... some people have reported it didn't flash without it. (Worked great for me, without flashing boot.img before install)
But for all other roms:
THE FIRST TIME YOU OPEN THE FLASH GUI APP UP IT WILL WARN YOU ABOUT NEEDING AN UNLOCKED DEVICE, JUST HIT "OK"
Open up Flash Img GUI app > browse> downloads> rom.zip> It will pop up a dialog warning you that the file size is larger than a kernel, and ask you if it is a rom... select yes
It will preload the rom temporarily into system files (might take a minute or two)> another dialog will come up (i think) just hit "OK", then hit the "Flash Image" button at the bottom... once this is done, it will tell you that you need to reboot. It will give you 3 options, select the middle one "Reboot to Recovery"... Once you get to recovery select Wipe>Factory reset. After the Factory Reset is done, back out to the main menu of TWRP and select Install> browse to "downloads" and select rom.zip and flash
Once it is done, select "clear cache" (most roms do this already durring the installation) then reboot system. First boot may take longer than usual (had one take a full 10 minutes one time) and it is possible that it will "Bootloop" once, but then it'll boot into the new rom. Yay! time to have fun haha