Use the wipe menu in TWRP to do a full wipe (cache/dalvik/factory reset/system). Then use the install option to the install the ROM (and gapps if it's an aosp ROM). Then reboot to bootloader and flash the boot.img with fastboot. Then reboot your phone.
Sent from my Evita.
i was unable to do a wipe of my phone. in twrp i went to wipe and "swipe to factory reset" it ran through and came back as failed..
XDA Developers was founded by developers, for developers. It is now a valuable resource for people who want to make the most of their mobile devices, from customizing the look and feel to adding new functionality. Are you a developer?