Mix the dry ingredients. In a large bowl, whisk together the citric acid, baking soda, washing soda, and Epsom salt.
Combine the liquids. In a small separate bowl, mix the hydrogen peroxide and water, then add the essential oil (if using) and mix.
Make the dough. Put your gloves on if you're using them. Slowly add the liquid mixture into the dry ingredients while stirring continuously. It's ok if the mixture fizzes slightly. It should come together into a "dough" that sticks together when formed in your hand.
Press into molds. Use a ½-tablespoon scoop to portion the dough, then firmly press it into a silicone mold. Muffin liners, soap molds, or ice cube trays work well, but silicone makes removal easiest once they're dry.
Let the tablets dry. Dry for 24–48 hours in a low-humidity area. A small room with a dehumidifier can cut this to about 12 hours. If the tablets rise while drying, press them flat with the back of a spoon.
Store the tablets. Remove the dried tabs from the molds and store in an airtight container.
Use the tablets. Run the washing machine with 1-2 tabs on a warm or hot water cycle, or the cleaning cycle if your machine has one. Wipe the inside of the machine with a rag or scrubby after the cycle if needed.