Successful pattern alteration demands changing size without changing proportions and lines. Correct changes are made by tucking or piecing patterns near their center and without changing the original size. It is not advisable to cut beyond a pattern or turn the edge of pattern since such alterations change its shape as well as its size.
After deciding where the pattern needs to be altered, draw a straight line as a guide for cutting the pattern. For most alterations make the line parallel cut along this line as needed, so that it will spread out flat. If your are lengthening or shortening an entire pattern piece, you will need to cut the pattern all the way across and increase fullness entirely through pattern. Before cutting, draw two short lines a few inches apart, at right angles to and across the cutting line. Then match these lines when you spread and place the pattern, keep the pieces in correct position.
Spread or place, the cut edges to make the pattern larger or smaller, as needed. When you spread the sections apart, lay a strip of paper underneath the cut edges. Check the altered pattern measurements to be sure that they are with the measurements, needed then paste, pin or use gummed tape to fasten the pattern pieces in place.
Sometimes more than one change is also needed in a pattern piece. The best way is to take care of each alteration separately. Try not to change the shape of the armholes, neckline, or shoulders anymore than necessary.
If the alteration is difficult, then make a duplicate pattern made with newspaper, make all markings and notches on the duplicate, and make the necessary corrections on it. It is a good idea to try out the altered pattern in muslin or old cloth to be sure of the fit. Then alter the original pattern in the same way.
Pattern alteration can be best done by a slash and spread method for enlarging a pattern and fold pleat for reducing it. While altering a pattern it is necessary that all grain lines must be straight.
Equipment needed for altering patterns, includes a pair of scissors, some light weight paper, and pins. Heavy paper is not suitable for making alterations, also a ruler and pencil to redraw construction details that are interrupted by your alterations are necessary.