Getting your measurements
You're going to need to know your measurements going in. There's this handy guide to taking your body measurements, you'll want to have these written down somewhere before you go shopping, specifically your chest/bust, waist, and hip measurements, and then use the sizing chart on the back of the pattern to determine what version you need to look up.
If you're like me, you probably don't look like the model on the right, and you might find that your measurements don't quite line up right--your chest/bust line might be in one size, your waist in another, and then your hips in a third. For a simple sleeveless tunic, you basically want to size to either your chest or your waist, and then go with the larger size (If your chest is a L and your waist is a M, go with the L sizing). This isn't really a hard and fast rule, different patterns might require different allowances, and later on I hope to get into adjusting a pattern to fit your particular physique.