The problem with commercial sewing patterns is that they're usually based on an "average" body. And while average may mean something when you're looking at an entire population, for individuals it's not so useful. How many times have you looked at a pattern and realized that all but one of the measurements is exactly what you need? You end up with something that fits perfectly except for being too tight across the shoulders, or that would look great if only it were two inches longer. Fear not, there is a solution!
That solution is pretty generic: look for a place where you can add a straight line. What that means is, find a place on the pattern where you can add a slice of fabric that isn't going to disrupt any of the shaping. Most patterns will have such a place marked, at least if you need to make them longer; it'll be on the pattern piece as "fold here to shorten" or something similar (for those of us who are short), but there's no reason you can't add a little extra length in the same spot.
If you need extra width, matters are likely to be more complex; the cardinal rule is to avoid the neckline and armseyes. This usually results in two vertical lines, one from each shoulder down to the hem. For a button-down shirt, each front will get one increase line and the back will get two. It'll change the slope of the shoulder very slightly, but that's not generally a problem. If your original pattern is in many pieces, such as a princess-seamed dress or gored skirt, your job is easy as pie: find how many seams you have that are reasonably straight-that is, that don't have armholes or necklines on them. Figure out how much extra width you need, and divide that by the number of seams you have to play with. When cutting, add enough extra to each "straight" seam to add up to your extra, and voila!
If you have to actually cut your pattern, matters are only slightly more complicated. Once you've figured out where on the pattern you're going to put the straight line, cut the piece along that line. Now you have two pattern pieces where before you had one. If you're planning to use the pattern again, take some butcher's paper or the like and arrange the pattern pieces on it so that whatever extra you need lies between the edges you cut. Tape the pattern down firmly, cut out the butcher's paper, and there you are. Hemlines, being generally fairly straight, are unlikely to cause problems, but you'll have to do a tiny bit of fussing to make shoulders come out right-it should be obvious just looking at it, but in brief all you have to do is even out the slope from the neck to the shoulder point. If you're not planning to make this garment again, you can pin the pattern pieces straight to the fabric for cutting out.
Of course you can add to a pattern in such a way as to intersect the armseyes or neckline, but doing so is liable to be much trickier and requires diagrams that can't be included here. Just keep in mind that any alteration of one piece has to be matched on any piece it's going to be attached to, and you probably can't go far wrong-but it might be a good idea to try your alterations out on muslin first.