Wear the slippers as you practice, but before you take an actual dance class. This allows you to wear the them for short periods of time and take breaks from them, which you can't do while participating in a class.
Pinpoint the areas of the ballet slippers that pinch or hurt. Apply athletic tape to your skin anywhere the slippers are rubbing painfully. This keeps your skin from being abraded, which helps you tolerate the slippers for a longer period as you dance, which, in turn, makes the break-in period shorter. Once the slippers are properly broken in, eliminate the tape.
Practice pointing your toes and standing on relevé in the slippers. This stretches out the shoe.
Run water over any area that feels particularly tight when you dance. This allows that part of the slipper to stretch and mold to your feet as you're dancing.