Boot into bootloader, connect phone to PC, open command prompt, change directory to adb.exe / fastboot.exe folder. Type fastboot devices to make sure your device gets picked up, should show you serial number of the phone. Then type fastboot boot name_of_recovery.img. Just make sure you have the custom recovery(TWRP) extracted(not in .zip form) in the fastboot/adb folder as well.
Done, that's it
Edit: Of course I'm sure you know the bootloader needs to be unlocked as well.
I also feel proud to be one of the few who knows how to get adb/flashing to work via my windows 8.1 USB3.0 only system - but that's another story.
thanks again for the response - going to go do this later today.