1) Phone should be on, as usual. Connected to PC via USB cable. ADB enabled, etc.
2) Open a terminal on your PC and issue your fastboot command, like "fastboot oem unlock" or whatever you want to do.
3) You should see something like "waiting for any device".
4) Reboot the phone.
5) Hopefully, fastboot will grab the phone in that brief moment when it's actually in fastboot mode.
I accidentally did this with a Google Pixel that was being difficult, and it worked for me.
Any other ideas?
I would imagine that its a combination of holding down the power button and going throught a number of reboot cycles, but i dont know.