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If your shoes are particularly dirty, you may want to start by taking them outside and slapping them together to knock loose any large bits of dirt or mud. If you leave too much mud on the shoe, it will take a lot longer to get them clean.  Make sure to slap the shoes together outdoors so you don’t make a mess inside your house. You may even want to use a butter knife or key to scrape set mud out of grooves in the shoe. Before you begin scrubbing away at the rubber parts of your shoes, start by brushing or even scraping off any loose dirt that’s stuck to it. The more you remove with a dry brush, the less mess you’ll have to deal with once you create your cleaning solution.  Don’t worry about scrubbing too much, if the dirt doesn’t come free quickly, it will once you begin using the cleaning solution. Use a dry brush like a toothbrush, but avoid steel bristle brushes that may damage the rubber soles of your shoes. Depending on how much you need to clean, you likely won’t need much baking soda or laundry detergent. Begin by mixing a tablespoon of each in a small bowl thoroughly. You can always add more of each ingredient if you find you haven’t made enough.  The baking soda will act as an abrasive to help the soap remove dirt and grime. Avoid using laundry detergents with bleaching agents. Use your brush to apply the combination of baking soda and laundry detergent to the rubber portion of your shoes, then scrub it in. Using a circular motion with the brush is often the most effective way to remove dirt and grime.  You may want to avoid using this cleaning combination on the fabric of your shoes, as the baking soda may be difficult to thoroughly rinse off. You can make a separate mixture of just detergent and water to clean cloth portions of your shoes. Once you’ve adequately scrubbed the cleaning combination into the rubber soles of your shoes, take another clean cloth or sponge and soak it in clean water. Run it along the rubber, rinsing it with each pass until all of the mixture has been rinsed away..  Failing to remove all of the cleaning mixture can leave the rubber looking discolored. Leaving the detergent mixture on the shoe can also make them very slippery and dangerous. Once you have rinsed all of the soap off of the shoe, use a clean towel to dry the rubber before you wear the shoes again. Once you have the shoe dry, you’ll have a better sense of how well the cleaning mixture did, and you can repeat the process at this point if you need to.  Leaving the shoes wet could lead to them beginning to smell. Wet shoes can be dangerous to wear, so be sure they’re completely dry and free of soap before putting them on.

Summary:
Remove any caked-on dirt. Use a dry brush to remove loose dirt. Mix one part baking soda and one part laundry detergent. Scrub the rubber with your cleaning solution. Use a different sponge or cloth to thoroughly rinse the rubber. Dry the shoe completely.