Thanks to Karin for this Frugal Baby Tip!
I'll have a go at describing the origami fold. You do need a square to begin with, so you're alright!
First fold up the diaper into four layers by folding in half twice, once to make a rectangle, and then again to make another, smaller, square.
I'm left handed so if you're not just reverse these next bits!! I never know if I'm doing things left-handed or right-handed!
Put all four corners of the diaper to the upper left hand corner (you're working on a flat surface). You now have one thick fold down the right hand side, and two slightly thinner folds across the bottom. Pick up the top corner. Keeping the second corner in its place, take the top corner across to the right as far as you can. The top part should now look as long as your rectangle that you made at first, and now you will have what looks like a triangle, with its center line along the right hand side of a square underneath.
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I hope this comes through alright!
Carefully flip the whole thing over, left to right or right to left (not flipping it upside down). Then you have the triangle on bottom, and the square on top, to one side. The triangle is the basic shape of your diaper, and you will fold the square to make a central pad. Fold the square, starting with the edge away from the triangle. Just fold over and over, adjusting your folding so you will end up with a center pad. You can fold it wider or narrower, as you desire.