Followed this and it worked up until I ran "fastboot boot twrp.img" which was the IMG file from TWRP RC1. The command line seems to say it all worked well, but the phone then restarts into the OS and never goes to TWRP?No link handy, but here is how I do it. And yes, it is via fastboot, only way I know of at this point.
Download 2.7.9 SR3 SuperUser and copy it to your phone.
https://forum.xda-developers.com/apps/supersu/2014-09-02-supersu-v2-05-t2868133
Download TWRP RC1 and copy it to your fastboot folder.
https://forum.xda-developers.com/pixel-xl/development/twrp-alpha1-pixel-devices-t3500312
Using other versions than above will bootloop you. Reboot the phone into bootloader mode. Power and Vol down.
fastboot BOOT twrpFilename.img Where twrpfilename.img is the name of the TWRP file you just downloaded. Note is says boot in the command, not flash.
That will boot the phone into TWRP. Select Install and install the SU zip you copied to the phone above. And reboot the phone, done.
Any thoughts, and thanks for the instuctions so far!!