Step 1: Initial fit check
If you're picking up a new sunshade for this project, get one that covers as much of your windshield as possible when you lay it on the outside. Sunshade manufacturers want to keep their costs down, so they produce the minimum number of models. I have only seen two sizes in the auto parts stores: regular and large. If you read the application notes on the back, you'll see a slew of makes and models allegedly covered. Ignore these suggestions and get the big one: you're going to make it fit, and the extra size will come in handy.
What you see below is my trusty 1987 Toyota Camry (affectionately called "The Death Trap" by coworkers) with a large sunshade. You can see that there's plenty to work with. The sunshade has shiny mylar on top of some sort of bubble-wrap-like material. If you want to keep costs down, and you're willing to do a bit of folding at the end, you can make your own sunshade from a big piece of cardboard.
Windshields tend to curve in at least two dimensions, which makes them tough to size using simple measurements. We need a template to guide our modifications.