Place the potpourri inside a sachet. Leave it there for at least one week. Do not leave it in a dark cupboard; instead find somewhere with fresh air and indirect, cool light. It's a good idea to leave the sachet in the bag when using as well, as the scent will continue to improve the bag's own scent.

Summary:
Make or purchase some potpourri. Place the sachet inside the smelly bag. Place the bag in an airy place. Check a week later.