From our in-house barber and trainer Sheldon how to break in your new Proraso Professional Shaving Brush.
Soak your brush overnight in a bowl of water with about a tablespoon of salt. This will help soften the bristles and neutralize any "new brush smell" from the natural bristles.
Make sure to rinse your brush thoroughly (this can take 3-4 minutes), then allow the excess water to drain. Hang your brush from a stand or shake it out and lay sideways on a towel.