I was told about this hack. I have heard about it more than once from different sources; but, I have never tried it myself.
To stretch (break-in) new leather shoes so that they are comfortable to wear, carefully line each shoe with plastic such as a thick sheet of plastic or use a heavy gauge plastic freezer bag.
Push the plastic into each shoe as far and as completely as you can. Lace up and tie the shoelaces securely.
Fill each plastic lined shoe with water up to the brim and freeze it SOLID in a freezer. The ice inside the bag in the shoe will take the shape of the shoe.
The water will freeze, swell, and stretch the leather proportionately as it increases size. When the water has frozen solid, the process is finished and the bag and ice can be removed.
The shoes should fit perfectly. If more stretching is wanted, repeat the process.