Take the liner out of the boot and step in (wearing your ski sock). Keep your heel in the back of the boot, maybe half a centimeter off the shell wall, and shift the inside portion of your foot to the inside portion of the boot. For a comfort fit, you should be able to get a finger (1cm) between the outside of your foot and the shell. For a performance fit, get a finger tip (5mm). Repeat for the inside portion of the foot. If at any point your cannot get a finger tip between your foot and the shell (meaning your foot makes direct contact with the shell) you will most likely need some shell work to some degree depending on your comfort/pain threshold.
Bromance Crew, since '08