Many modern boots comes with a type of liner boot that is designed to be heat-molded to your foot. These boots are sized very tight in the store, so that your foot can barely fit inside them; after the liner boot is heat molded, the fit will feel snug, and after a few days of skiing on them, you will have a precision fit. Most ski shops will offer to mold your liners when you buy the boots, but if they aren't equipped to do, it is something you can do at home.
Verify that the boots have a thermoflex liner, not a thermofit liner. Thermofit liners require a special fitting that can't be done at home; thermoflex liners can be molded using a conventional oven.
Create a toe cap by cutting pieces of foam and placing them between your big toe and second toe and between your last toe and fourth toe. Cut a heavy wool sock so that it slips over your toes and stops at the end of your toes. Put on your normal ski sock over the toe cap.
Bake the liners in an oven that has been preheated to 325 degrees, then turned down to warm. Place foil over the rack to avoid burning the liners. Bake them for six minutes, and check after four minutes and five minutes to make sure they haven't burned.
Remove the liners from the oven, insert your custom footbed into the liners, then slide the liner into the boot. Lift the tongue of the boot and place your foot into the liner. Fasten the clasps on the boot and tighten halfway.
Stand on a moderate incline, no more than five degrees. A book or a two by four will suffice. Distribute your weight to your feet evenly while the liners cool. Allow about ten minutes for the liners to mold to your feet.