Lawn mower equipment needs to be maintained so it lasts. This includes the grass clippings bag that attaches to the back of your lawn mower. Cleaning it on a regular basis makes it last longer. Wet grass clippings left in a nylon mower bag mold and smell bad.
Stop the lawn mower. Remove the bag and empty the bag of all the grass clips. If you do not know how to remove the bag, check your user manual.
Check the interior of the bag to make sure the grass clippings have been removed.
Spray the interior and exterior of the bag with a garden hose. Scrub the outside and inside of the bag with a soft brush.
Rinse the bag again with the garden hose. Rest the bag in the sun. Try to expose the interior to the sun. Let it sit until it's dry. Store in a dry location.