it's important to make sure that the boots fit properly

Before you rent or buy ski boots, it's important to make sure that the boots fit properly. The next step is to make sure that your boots are adjusted for optimal skiing.
Try Before You Rent or Buy
Like shoes, not all ski boots that are the same size will fit the same way. Some brands of boots have wider widths, others run larger or smaller. Try your boots on with the socks that you will be wearing skiing. The boots should feel snug on your foot. Make sure that the boots are comfortable. There's nothing much worse than skiing all day on boots that make your feet sore. You may have to try on several pairs to find a boot that fits correctly.
Entry Point
Ski boots will either be top entry or rear entry. With rear entry, you slip your foot into the boot from the back.
With top entry, insert your foot from the top.
Tighten the Clips
Once your feet are in the boots, you can latch the clips to tighten the boot. Rear entry boots typically have one clip, while top entry boots have as many as four clips you can use to adjust the fit of the ski boot.
Tighten the Power Strap
Most ski boots have a velcro strap called a power strap, across the top of the boot. Pull it tight to adjust the fit of your boots, so they are tight on your calves and shins.
Once Your Boots are Buckled
Once you've tightened the clips and the strap, make sure that you're comfortable. You don't want the boots to be too tight (and cut off your circulation) or too loose, which will impede your skiing. Be sure that you have room to move your toes. When you bend your knees, make sure that your heel stays put and your foot doesn't move up. If your foot is sliding in the boot, rebuckle it so it's tighter.