Unlike the previous folds, this step does not rely on definitive markers to execute the fold. This is often referred to as making the fold 'to taste'. Turn the model over and make a fold on the side to match the preceding fold. You now have the model. By tapping the folded 'spine', you make the creature jiggle across the table.
Image is a picture taken with a digital camera with additional lines added for clarity.
An appropriate answer would be 'squares and triangles'. The answer is yes, but it may not seem that way because of our orientation to the paper. The answer is the length of the diagonal of the original paper minus two finger-widths. The answer is not that definite. It is the height of a triangle with two equal sides, each the finger-width, but it depends on the sharpness of the angle.