Put the washing soda in a glass bowl and mix in the salt and citric acid.
Stir in the essential oil, pressing out lumps with the back of a wooden spoon.
Spritz in the water a little at a time while stirring (adding it too fast makes it fizz), until the mix feels like slightly damp sand.
Pack it into a silicone mold.
Leave it to harden and dry overnight.
Pop the tabs out and keep them in an airtight container with a label.
Notes
Use one tab in the detergent cup per load.Safety: dishwasher tabs are not candy, though they look like it. Store high, closed and labeled, away from kids and pets; don't use a mold or container that looks like a treat tin. Wear gloves when pressing.