Welcome to the forum!
Good choice on book, you can make some fantastic bouquets from it! Step ten is pretty tricky, but hopefully this will help.
The first thing to make sure is that your creases for the middle are
sharp. Go ahead and stand it up so that the square is by itself in the middle.
Start pinching one of the corners so that you collapse half of the square.
Now start pinching from the other side to finish collapsing the middle square.
Now you can finish the model (and do this step three more times!) This is a great stepping stone towards more complex models; the collapse in step 10 is a precursor for the dreaded open sink. Let me know if I need to explain something better, or explain collapsing the middle differently, this is just the easiest way that works for me.
Have fun!