Below are the pictures I've taken of the folding steps... sorry if it looks more confusing with the arrows.
1- fold in half, taking the length as your base
2- fold another half
Now that you have a square (or rectangular), lift one layer to create a triangle as in picture #3 below.
4- Flip it over. This is how it looks like. Continue folding.
It wouldn't be a bad idea to fold the diapers beforehand and keep them in a basket for future use.