https://lineage.microg.org/
It's pretty simple if your phone is supported. You enable developer mode, enable USB debugging, download the image, and run a couple ADB commands. For me it was:
fastboot flashing unlock
fastboot update image.zip
And that was it.
Not GP, but I'd been using LineageOS on my HTC OneMax. I like it a lot.
Here's a list of supported devices:
https://wiki.lineageos.org/devices/
Click through the links on the above page for install guides.