Updated February 2016
boot into TWRP
Regular user with a new rom.
click wipe button, and click the factory reset slider 3 times then click home button, and reboot into recovery before flashing, that should be all you have to do.
Power user or having problems after the process up above, warning this wipes the phone!
For a full wipe Format Data also.
and factory reset
I do all these 3 times