The yellow square below can be folded into a single square in many ways. For example, it is relatively easy to fold it so that the sequence of squares from front to back is 123654789. However, it can quickly get very complicated if you start tucking in squares in little holes, getting sequences like 231469785.

1 | 2 | 3 | | | | | X |

4 | 5 | 6 | | | P | | S |

7 | 8 | 9 | | | U | | |

It is possible to fold the blue square above so that the letters on the squares from front to back spell PERPLEXUS, provided that you fill the unlabeled squares with the rest of the letters. How should you fill in the unlabeled squares?