First of all, welcome to the forum!!
This is a bit of a misleading step. The book calls for a squash fold, but in reality, it's a spread-sink. Here's a few reference points:
If you start after step 15, swivel-fold the top part to put a vertical crease on the right side.
Now on step 16/17, you'll wind up making a new crease. Flare the top out using the creases you just made and massage the paper to spread-sink the top half.
Hopefully this helps! Let me know if you have any other questions or if you're still confused on what to do. Have fun!