Commercial sewing patterns are the home sewists dream: readily available at big-box craft stores, extremely affordable when purchased on sale, hundreds of options in almost every conceivable silhouette. And if you happen to be sewing for a figure with measurements that match the pattern measurements, the situation is pretty much ideal!
However, most people don't have measurements that match what the pattern companies use. Depending on the degree of alterations needed, home sewists have several options when it comes to getting around this roadblock: working from a muslin to finalize fit before sewing the actual garment, altering the garment after it's been sewn (only good for very minor adjustments, such as a hem that needs to be taken up slightly), or altering the pattern before cutting the fabric.
Altering the pattern before cutting is less time consuming than working with a muslin, but what will work in any situation will depend on the degree of alterations needed.
One of my recent sewing projects was a wrap dress. Based on my measurements, my bust fell into a size 8 while my hips were a size 12. That's a pretty big difference, even though a wrap dress has a more forgiving fit than other styles. Fortunately, the bodice and the skirt were separate pattern pieces. I cut the pattern in the appropriate size for the top and bottom and relied on altering the pattern prior to cutting the fabric to get the seams to match up.
To do this, I measured the difference between the bodice back and the skirt back. It was about 1". To maintain the lines of the pattern, I split the difference by 2 (so, 1/2") and created small darts on either side of the skirt. It does take a bit of smooshing to get the rest of the pattern to lay flat once you create the dart, but this is a very effect method for altering commercial patterns for width issues. Depending on the construction of the garment, you will want to repeat the dart process for all pieces. In my wrap dress example, the bodice and skirt fronts were each two pieces, so I only needed to create one 1/2" dart on the skirt front. The best part of this method is that you can easily check your alteration: if the width of the pattern pieces now match up correctly, you've successfully altered the pattern.
Another common fit issue that can be easily solved by altering commercial patterns is length. When altering patterns for length, I suggest making a copy of the original pattern first. This way you can cut and mark up the copy, and if things go awry, you still have the original intact and can start over.
If you need to lengthen (or shorten) a pattern by more than 1" or so, it's best to spread the difference across the pattern. For instance, if you need to lengthen a pant leg by 2", you would add an inch in the thigh area and then the other inch toward the bottom of the leg. This is a very similar method to grading a pattern up or down; you can read more about pattern grading techniques here.
If you're interested in learning more about altering patterns to fit your body, check out the Craftsy online sewing class Sew the Perfect Fit.