A good way to start (other than regularly visiting the Monastery) is to dig out the original/amended/updated specs of this project, so you can find out what it was meant to do and (perhaps) how it should look.

This may help you to understand the program (if it was written to conform to the specs, that is) and more easily find your way into it.

It may perhaps even show you that it is better to start over from scratch. This may seem a daunty task, but with the help of some templating systems (Template-Toolkit or Mason or another templating system (super-search the monastery to find more) it may even be less demanding than finding your way into the undocumented sub-standard code of someone else and at least it let you adapt future versions of this script with less effort.

Oh yes, and if you found the original specs, see if you can find how much the consultant charged for his "work". Perhaps you can negotiate a bonus along the same lines once the job is finished ;-) and don't forget to drop a small bit of your bonus in our Offering Plate.

CountZero

"If you have four groups working on a compiler, you'll get a 4-pass compiler." - Conway's Law