First, make a back up of your stock rom by hitting the back up button (if you still have it)
Then select "Wipe", do a factory reset
Next reboot to bootloader, and connect to pc
Using cmd prompt, "fastboot boot boot.img" (make sure the boot img from your selected rom is in that ADB folder), after it gets done with that (which will be relatively quick) reboot into recovery.
Now go to install, select the rom.zip that is on your phone, and flash it... Reboot, and it should boot up into your new rom. (This first boot may take some time, so be patient and just wait-- had one take over 5 minutes one time)
Now if you haven't put the rom.zip on your phone yet, you should still be able to open up your phone on your PC like it is a jump drive, just drag and drop (if you can't see your phone as an available storage device on your PC, reboot to the bootloader and see if it is in fastboot -- it should say toward the top of the screen, then turn fastboot off by selecting one of the choices [use volume buttons to go up and down on the menu and power button to selext]). It might be that it needs to be IN fastboot to see it as a storage device, I can't remember, but just turn it off or on until you can access it