Current builds (starting with r21) are only for Lollipop 5.1.
Builds (starting with r18 up to r20) are Android 5.0 Lollipop only.
KitKat is dead and the last stable release was r17, which you're free to use if you're still on 4.4.4.
fastboot flash boot boot_name_img
flash the .zip in the recovery
Got any questions that you couldn't find after searching?
Disclaimer to moderators:
Slight OT is fine, but when users decide to act stupid they should be dealt with with the appropriate means - its really annoying for me when OT turns crazy. Please put the warning/ban hammer anytime any of you think there's lack of respect and/or too much unwanted OT crap - but if it doesn't meet this criteria let the OT posts alone. Thank you.