How to Alter Sewing Patterns. While there are many ways to alter sewing patterns, the easiest and most highly recommended for novice seamsters is the 'seam method.' The seam method places all alterations to patterns as close as possible to the seam line of the fabric, which can significantly reduce the possibility of error.
Use the Seam Method to Alter a Sewing Pattern
Take accurate measurements of the person for whom you are making the garment. Make sure to measure every part of the person's body the garment will cover, both lengthwise and widthwise. Allow for any tapering or broadening of pattern lines you want to make.
Prepare a clean, flat, spacious work surface. A large, round sewing table works very well for altering sewing patterns.
Iron the paper pattern using moderate heat and your iron's steam-free, dry setting. This will keep it flat and in its proper proportions for cutting.
Using sharp scissors, make clean cuts along the lines of the paper sewing pattern. Make sure you don't alter the pattern's seam allowances in any way.
Lay the pattern atop the fabric. Use pins to secure it if you would prefer to do so.
Trace any alterations to the length that will be represented equally throughout the pattern onto the fabric using a pencil. Then, proceed to trace any alterations to the length that will be unequally represented.
Repeat Step 6 for any changes to the width of the pattern. It is imperative to execute Step 6 and Step 7 in the given order, to the letter, to prevent any possible disproportion in the finished garment.
Double-check when you've finished tracing the alterations onto the fabric to make sure that the changes you're making correspond to the measurements you took in Step 1. Make sure that your seam lines on both the front and back center of the pattern remain intact.
Lay the pattern flat. Any bunching up of the fabric will mean that the garment will be smaller than intended in the bunched up areas. Prevent this by taking great care to ensure the accuracy of your work in Step 6 and Step 7.
Make your alterations beginning at the top of the pattern (which will later become the top of the garment) and working your way down. Correcting for length will not affect the width, and vice-versa, so it is important to work from the top down.