I started out with these ingredients, and Rhonda Jean's recipe from her book, although it is on her blog as well at http://down---to---earth.blogspot.com
There are four bars of soap in that pack, and I needed to use two of them.
So I grated 2 bars of soap, with an old cheese grater, and then put it in a bowl with 2 cups each of borax and the soda crystals. I gave it all a jolly good stir and put it into a tub with a lid, and included an old tablespoon measure which I can keep specially for this purpose. Two tablespoons per load is the recommended measure.
I have done two loads of towels with this new mixture, and not only have they come out clean as required, but they smell faintly of soap, which is quite pleasant. I didn't add the essential oil although I might do in future.
And the cost? Very impressive!
The pack of soda crystals cost 99p for the 1kg, and I used 350g, so a cost of 35p.
The borax pack is £1.29 for 500g, of which I used 400g, so that cost £1.03.
The four pack of soap currently costs £1.33 at Tesco, and I used two of the four bars, which cost me 66p.
Total cost is £2.04 for the batch of laundry powder. I then did the boring but predictable thing - I measured how many loads at 2tbsp each I would get from the batch and realised that 30 would be about right, hence a cost per load of just 7p.