You only need to unlock the bootloader and have a custom recovery to flash. The ROM is rooted, so rooting before flashing it is optional. You can find links in the Unlock guide to either TWRP or CWM recovery.. I recommend TWRP. Download the ROM and GApps and put them on the phone (after unlocking, as unlocking will wipe everything, sdcard included). Then boot into the recovery using fastboot commands and within it:
Wipe data/factory reset <--- Won't wipe the sdcard partition
Install the ROM
Install the GApps
Select Reboot system.
Then enjoy