There are four ingredients in the Pad Thai sauce, Tamarind pulp (for the sour flavor), Fish Sauce (for the salty part), Palm Sugar (for a slight sweetness), and Paprika or Thai chilli powder (for the spice). Two cups of sauce will make about 6-8 portions of Pad Thai. You can make your Pad Thai sauce vegetarian by using this sauce instead of fish sauce.
To make about two cups of sauce, you should begin with about ½ cup each of Tamarind (*see the note below for how to prepare tamarind pulp), Fish Sauce, and Palm Sugar. If you substitute white and/or brown sugar for the Palm Sugar, you should use only about 1/3 cup. Melt all these together in a small pot over a low flame. Taste and adjust the flavor balance until it suits you. Then add the chilli powder, begin with a teaspoon or two, depending on your taste, and keep adding until it tastes the way you like it. By the time you're done flavoring the pot should be simmering happily. Turn off the heat and let the sauce rest while you get to the other ingredients.
At this point in the game I like my sauce to lead with a salty flavor, follow by a mild sourness, then just a gentle sweetness and a soft caress from the chilli at the back of my throat at the very end. A finished plate of Pad Thai will be served with a sliver of lime and extra chilli powder to be mixed in at the table, so you could keep these two flavors in the sauce mild for now. I don't know about you but there is nothing I hate more than a cloying sweet Pad Thai. If your sauce starts out super sweet now it will be very tough to correct later.
Those of you with a scientific mind might want more precise measurements or proportion or whatnot. I'd do it if I could, but the problem is most Thai ingredients are not standardized in the way that a Western ingredient, say, white granulated sugar, is. A cup of granulated sugar is always the same, but a cup of your Palm Sugar or Fish Sauce might not have the same intensity as mine. So the easiest thing to do is just to taste. And herein lies another beauty of preparing the sauce ahead of time. You can take your time to taste and adjust the sauce precisely to your liking, which would be hard to do à la minute in the wok.