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Mix the baking soda and water in a bowl until they are well-combined. You are looking for a mixture with a toothpaste-like consistency. If the mixture is too thin, then just add more baking soda until a thick paste forms. Let the paste set for 10 to 15 minutes. For tough stains, let the paste set for longer like 30 minutes to an hour. Use a scrub brush to do this after the paste has set for the allotted time. Scrub in a vigorous back and forth motion until they are gone. Use a toothbrush to clean small crevices. If the stains remain, then you may need to repeat steps one through three again. Wipe until any and all paste, debris and grime are removed. Then use a clean, dry towel to wipe your shower down until it is completely dry.

Summary:
Mix 3 parts baking soda to 1 part water to form a paste. Apply the paste onto the stained areas with a sponge. Scrub the stains. Wipe the area clean with a damp towel.