Summarize:

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