To flash a ROM you need the ZIP on a directory on you phone AND the boot.img file in your fastboot directory. Boot TWRP wipe everything except External Sd twice, and install the ZIP (and Gapps if required) from TWRP. After that reboot into bootloader and start fast boot and from your PC use 'fastboot flash boot boot.img' then reboot and your good to go. Remember first boot on most Roms will likely seem to take forever, so just be patient.
Sent from my Nexus 4 using Tapatalk