Empty the bag by checking all pockets and corners.
Fill a bath with plenty of luke warm water.
Add a small amount of detergent free soap. Do not use regular detergent or softener which can damage the material, especially if the bag is waterproof.
Using a soft brush or cloth scrub the bag, focusing on areas that are particularly dirty. A toothbrush is useful for particularly tough stains and getting into difficult to reach areas.
Allow the bag to dry naturally away from direct heat sources. If possible hang the bag upside down to dry. Do not tumble dry as this can damage the bag. Ensure the bag is 100% dry before storing.