Step 1: Parts Layout
(2) a second resource for showing how to do this origami: http://www.fishgoth.com/origami/basics5.html
Step 2: Square your rectangle
hopefully the pictures on this slide show what's going on.
Step 3: Fold and label square
Then, label the ends of each line as shown in the picture. (these labels can be "mental labels," for book keeping)
Step 4: Begin to fold...
fold points 1 and 2 onto 3 and A to A' and B to B'. Do it in one fluid motion and it will look like the picture.