First, gather the 4 little central flaps and stand them straight up, as diagramed in step 12. You may put a paper-clip on them, to keep them together.
Next, look at diagram 13. Slip your index fingers under the gray triangular regions marked with little dots. With your thumbs, push down at the two places where the horizontal valley-fold line meets the two-headed arrows. The bottom half of the bill swings upwards and around to the rear. This procedure is similar to a closed sink. (If you're not using a dollar bill, and the paper is weak, it might tear a little. This problem never happens with a fresh dollar bill.) Remove the paper clip.
Now you're at step 14. In this diagram, we are looking down at the model from above. The lower half resembles a little volcano (well, to me, at least). Flatten the model according to the valley-folds shown. Finally, spread the 4 little central flaps, 2 to each side. The model should now look like diagram 15.