8. Now that you've gone over all of that, will you finally tell me what pattern size to buy?
Yes! The first thing - and one of the most critical - things to understand about pattern sizes is that they have NOTHING to do with your clothing size. A person might wear a size 6 in jeans, but her size in Simplicity patterns might be a 10, and her size in Butterick patterns might be a 12. Or 14. Throw out, at once, the idea that buying a size 12 pattern means you are "fat." That is simply not true. 12 just happens to be a number. What is more important than that number 12 is that the garment, when finished, fits you, and fits you well. A plus-sized lady in an outfit (costume or otherwise) that fits her well will *always* look better than a thinner woman trying to squeeze herself into something a size too small (ever notice that slim girls still get muffintops when they wear pants that are just way. too. tight? It has nothing to do with weight!).
Take a look at the "body measurements" section of the pattern envelope. Typically, patterns will list bust, hip, and waist measurements - if you have these on hand before you go to the store, it will make your life easier. Look up a tutorial on the appropriate way of taking measurements, and don't pull the tape measure too tight. Remember, you're not trying to impress anybody with a tiny waist number and a big bust number - you're trying to make sure the pattern you buy will be the right size for your dimensions! For a skirt, the hip measurement is probably the most important one: you can adjust the waist as necessary, but if the skirt is too narrow to fit your hips, there's not much you can do about it. On the waist measurement line, look across horizontally until you find your measurement or the next highest number. For example, a 35 1/4 inch hip measurement lands between 34.5 inches and 36 inches, so bump up to the 36. If you follow that column down 2 lines, you'll be on the line that reads "Pattern Size." A 36 inch hip corresponds to a size 12 pattern size, as outlined by the blue box in the picture. (Note: You can use the waist measurement, too, or both together, to make your pattern size decision. There is a minor amount of trial-and-error here.)
Keep in mind that pattern makers design their patterns to ONE standard set of measurements - measurements that might not be all that relevant to the variety of body types of modern women! Looking at this pattern, it assumes a 10-inch difference between the waist measurement and the hip measurement. I don't know many women with those dimensions. This may mean you'll have to do some custom tailoring later on; another good thing to check against is the "Finished Garment Measurements" listed at the bottom. For example, a 32-inch waist measurement on this pattern translates to a size 18, which makes my jaw drop open - the finished garment measurement is 50 inches in the hips. Most of the people I know with a 32 inch waist would be swimming in that! The upshot is, the measurements are a good starting point, but be aware that they aren't a perfect indicator of size or of whether the final garment will fit you. The good thing is that nearly every commercial pattern nowadays is made to be somewhat customizable, so you can take in a bit here or add a bit there and you'll end up with something far more in line with your measurements. This is something that takes some practice, so don't fret about it too much when you're just starting out.
9. You've totally terrified me.
Don't panic! It's really not something to stress over. Generally, the basic measurement guidelines on the pattern envelope are sufficient for simple patterns for beginners. If you're really worried that your garment will be too small, go for the next pattern size up, and you can always take it in later.