Few of us are lucky enough to pull a pattern out of the envelope, sew it up, and have the finished garment fit perfectly. Learning how to alter sewing patterns can tremendously improve fit when you're sewing clothing. Here is a short tutorial to help you with the basics of adjusting patterns to fit.
One common alteration many people need to make is to either add or remove length to sleeve or pant patterns. Unless your pattern pieces are perfectly straight, it's not a good idea to make this pattern alteration at the bottom of the pattern piece. The reason is that you want to preserve the hem shape that is often at the bottom of these pattern pieces.
Here's how to lengthen pant legs or shirt sleeves:
If there is a "lengthen or shorten here" line marked on your pattern, use it! If not, go ahead and draw one on your pattern.
Cut the pattern along the lengthen/shorten line and tape tissue paper to one side of the cut pattern only.
Align the marked grainline of the two pattern pieces on a horizontal line on your cutting mat, measure the amount you need to add, and draw a line on the added tissue equal to that amount.
For example, if you need to add 1 inch, draw a line on the new tissue 1 inch from the lengthen/shorten line and tape the remaining cut pattern piece.
Finally, draw a new line along both side seams to form a shape similar to the original pattern. This may require that you add a little tissue under the side seam to accommodate the new line.
How to shorten pants legs or shirt sleeves
Note: Shortening pants or sleeves follows some of the same steps as lengthening.
Use or make a "shorten" line on your pattern piece, and fold the pattern along that line. At this point, I like to place the pattern piece on my cutting mat and align the pattern grainline on one of the grid lines on my mat.
Fold the pattern along the lengthen/shorten line, and fold out half the total amount of length you want to remove.
For example, if you want to shorten your pants by 2 inches, fold over 1 inch on your pattern piece. Be sure to keep the grainline on both sides of the fold straight along the grid line, and then tape down the pattern adjustment.
Lastly, draw a smooth line along the side seam line to form a shape similar to the original pattern. Congratulations! You just shortened a sleeve!
You can learn more garment adjusting techniques in Angela Wolf's class Tailoring Ready-to-Wear.