Add citrus essential oil/s, then mix and use spoon or spatula to break up any clumps
Spray water into mixture, stirring as you add the water. Don’t add the water too quickly or the washing soda and citric acid will fizz and react too much - slower is better
Stop adding the water once the mixture is the consistency of slightly damp sand. The mixture should clump together when you squeeze it, but fall apart if you drop the clump into the bowl. If it’s too wet, don’t worry, you can add more washing soda to dry it out. If it’s not wet enough, spritz it with a little more water.
Scoop the mixture into the silicone mould or ice-cube tray. Use the back of a wooden spoon and your fingers to firmly press the mixture into the mould
Let them dry and harden overnight
When they’re completely dry, pop them out of the mould and put them into an airtight container