Things You'll Need:
Tamarind paste (ideally, or else lime juice at a pinch)
raw sugar or brown sugar (even regular sugar will do)
fish sauce (or soy sauce or salt)
cayenne seasoning (if you like it spicy)
The secret to Pad Thai sauce is the interplay of four flavors: sour (tamarind paste or lime juice), sweet (sugar), salty (fish sauce, soy sauce or salt) and optionally spicy (cayenne seasoning or other pepper). All these ingredients are usually commonly available, with the possible exception of tamarind paste. But even this is available at most Asian grocery stores. Fish sauce has become a standard item in the international aisle of most mainstream grocery stores (in the Asian section)
There is no standard proportion of mixing these ingredients. I would adjust according to desired taste. For a single serving, I would start out with two tablespoons of fish sauce (or its salty equivalent). To that, add two tablespoons of sugar, half a tablespoon of tamarind (or 1 of lime juice), and optionally, 1 teaspoon of cayenne pepper seasoning (if you want a spicy edge).
Stir and microwave for 30 seconds, and stir again to mix, and then taste. Adjust ingredients for taste until the interplay is just right. If using tamarind paste, the sauce when hot should be a thick liquid, but not a paste.
Use your favorite Pad Thai recipe to prepare the dish and add the sauce you just made at the appropriate step. Any unused sauce can be refrigerated or frozen
Tips & Warnings
Getting the right consistency and taste to your sauce is an art, and it might take many rounds of mixing, heating and tasting until you get it just right
Tamarind paste can be overpoweringly sour, but it adds a kick and flavor to the sauce that you just can't get with lime juice. Go easy on the tamarind at first, and keep adding it and the sugar in turn until they go well together
Even if the sauce tastes very strong by itself, keep in mind that the flavor will get diluted when you add it to the noodles. So I would make it to be on the strong side, and then adjust the amount you need as you add it to the noodles.