I'm not sure I'm doing it the easiest way, but my workflow is this:
Remove bass from backing track with Moises
Export track without bass as .wav
Put track into Garageband
Fire up Quicktime video recorder and record using webcam (2012 iMac hence crap resolution)
Hit record in Garageband
Stop recording on Quicktime and Garageband
Drag video file into Garageband and line up (annoying)
Export as video file
I'm playing from my pedalboard into my Roland VB99 which is acting as a USB interface and I guess a sort of pre-amp (I'm using some cab sim and slight EQ)
There must be an easier way