If you take a look at the right-hand side of this photo, you'll actually see a previous (and successful) surgery, the stem all healed up, the twine suture still intact. Now, I don't mean that you should rush out and start playing Medic, but for all you squash and zucchini growers out there I do recommend getting down on your hands and knees and inspecting the main trunks of your plants for signs of squash vine borer infestation. Look for wilted leaves and little piles of sludge along the trunk that look somewhat like wet sawdust. Basically, you're looking for an entrance wound and some gross stuff. If your plants have been "compromised," don't worry. You can do surgery.
This hack-job work would not get me through med school, but I know my plant and I know that my plant will tough it out and survive. Once you open the main stem, you are in business. Slice all the way through so that you can splay the trunk open and scrape around in there. The idea is to locate the enemy and destroy him. The borer can be a clever foe, so be sure to find all of them while you're in there. Once that's done, it's time to suture up your plant. This is the fun and easy part.
I like to rub some dirt onto the outside of the wound, you know, like the dirt is some sort of topical plant antiseptic, which it probably isn't, and thus I mostly do it to complete the fantasy that I'm some sort of war-time, field medic, like Hemingway. Anyway, be sure to make the sutures real tight. There's no point in getting in there and getting all scratched up (the plants are spiky and they will scratch your face) if you plan on doing a crappy job in the end.
Hope this helps!