Gather your ingredients. You will also need 4 quarts of water, a large pot, a whisk, a container to store your finished detergent in (such as a clean gallon milk jug) and a funnel.
Liquid Castile soap can be a bit tricky to find...Trader Joe's and Target (in the beauty aisle) both stock it, though the best prices can often be found online. I found mine for around $16 at Target.
Bring 8 cups (2 qts) of water to a boil in a large pot.
Measure 1 1/2 cups washing soda (not baking soda), add to pot & mix with whisk. You can find Washing Soda in the laundry detergent aisle of most supermarkets.
Measure 1 cup Borax, add to pot & mix with whisk. You can also find Borax in the laundry detergent aisle of most supermarkets. This creates a bit of dust when pouring; you may want to wear a mask.
While Borax has excellent cleaning power (also, it is a natural mineral mined from the ground), research online first to decide if using Borax is right for you.
Measure 1/2 cup liquid Castile Soap & add to pot & mix with whisk. Choose whatever scent you like; I really like eucalyptus for dishes! For more aromatic detergent, add a few drops of essential oil.
Mixture will be frothy.
Reduce heat to low and add remaining 8 cups (2 qts) of water, mixing well. For a thicker detergent, let simmer 20-30 minutes. For thinner, more liquified detergent, skip the simmering.
Remove from heat and let cool to room temperature, stirring occasionally with a whisk.
Once completely cooled, transfer detergent into container. It's a bit easier to manage if you first pour it into a pitcher, then pour from the pitcher to the funnel.
The detergent should be thin with a very light amber hue. It's best to leave room in the container - it will expand when you shake it in the morning!
Let set overnight (out of reach of kids!) with the lid off.
By the following morning, the detergent should be "gelled". Put on the lid & shake well. After shaking, the detergent should be thick and white (and increased in volume). It is ready to use!
Yields approx. 1 gallon. Use 1-2 tbsp per load. Mixture may separate between uses and may be slightly irregular in texture; shake well before each use.
If you're making your first batch, try a 1/2 or 1/4 batch first to get the hang of it! While this recipe works like magic for me, recipes for dishwasher detergent are by no means "one size fits all"