There's probably no need to remount the bindings on the ski plates. Look at the outside heel area of your boot to find it's size in mm.
Remove the large screw on each binding beneath the ball of your foot. This holds a plastic toe piece in place. Beneath the plastic piece is where you'll be doing your adjusting. The heel piece is attached to the long metal part between the toe and heel, and slides with it. Round your boot size up to the nearest 10mm, and move the heel piece back so that the end of it is even with the mark for that size. You'll need to lift the end of the metal part up so that it doesn't catch on the hole in the toe piece. Once you have the correct sizing, replace the plastic piece and the screw.
Now click in your boot. The adjustment screw should be slightly recessed into the heel piece of the binding. If not, screw it in or out so that it is. Done.