Yep, it's 250g of washing soda (cheap in Wilkos or old style hardware stores), one bag of soap (make sure it says soap, not beauty bar, but nice smelling ones are fine or you can use Asda value), plus 4ish litres water.
Dissolve grated soap in hot water on the stove, then stir/whisk in the soda. It'll feel weirdly heavy after you add it. Then pour it into a tub to set as it cools.
Put a scoop into each wash, direct to the drum.
Hypo allergenic, doesn't leave a smell on your clothes (very important to my ASD kids), and cheap.
You can add a few drops of essential oil, or use perfumed soap, or drop in a free sample of perfume if you're feeling fancy 😁
The amount of water is flexible. I've used between 4 and 7 litres depending on the size of tubs I have available.