I am in the process of figuring this out right now too. In my case o have a 14" diameter plate I need to face and machine. I have a tormach 18"x18" x 10mm aluminum palate fixture but since this part needs to be nuts-on precise, I am planning on mounting it to my 4th axis in the horizontal position. I will use the superfly to face the part by moving the 4th axis.
If you don't have a 4th, I think you need some kind of fixture plate you can flip with reference holes or pins to reference your part.
Good luck.