How do you adjust a pattern?

I keep finding the most adorable patterns for tops out there. So, I’ve decided to knit myself a new wardrobe. My problem is the sizing of the patterns. Most patterns consider the XS size to be a 34" bust. But I’m a 30". How does one go about adjusting the size of a pattern?

Look at the stitch numbers given for the different sizes, usually there’s about 6, or 8 sts difference in them. Find out how many sts per inch you’re going to knit at, multiply that times 30 (or 32 for a loose fit, 28 for a snug fit) and that’s how many sts you need to fit at the largest point. Then see if that is one or two sets of 6/8 sts less than the smallest size. Use that ratio everywhere in the pattern where a st number is given.

I hope I’ve explained that clearly enough, ask if you don’t understand something.

That makes a lot of sense. Thanks so much!