This section is from the book "Clothing For Women: Selection, Design, Construction", by Laura I. Baldt. Also available from Amazon: Clothing For Women: Selection, Design, Construction.
Draw a line straight down from the center of the shoulder, through the waist line on both pieces of the pattern. Cut through these lines and separate the pieces enough to give one-quarter of the whole amount needed on both back and front (Fig. 87A and B).
Lay a fold extending from the center of the shoulder straight through the waist line on both pieces of the pattern, the fold to take up one-quarter of the entire amount the pattern needs to be decreased (Fig. 88).
Cut through pattern about two inches above the waist line; separate the pieces enough to give the added length, being careful to straighten the underarm seam line
Fig. 88. - Alteration of waist patterns; A, B, decreasing bust measure; C, decreasing width of sleeve.
Lay a fold through the pattern about two inches above the waist line on both back and front, the fold taking up the extra length. Straighten the underarm seam line by turning edge of pattern back on itself (Fig. 90A and B).
Fig. 89. - Alteration of waist patterns; A, B, increasing length of front and back; C, increasing sleeve length.
Fig. 90. - Alteration of waist patterns; A, B, decreasing length of front and back; B, decreasing sleeve length.
When the figure is extremely long and full in the bust, it is sometimes necessary to lengthen in two places, across the chest and back, as well as at the point above the waist line.
Apply the same principle for making wider or narrower, longer or shorter, as has been suggested for the waist (Figs. 87C, 88C, 89C, 90C).
Cut lengthwise through the centre of each side gore and separate the pieces sufficiently to give in all, one-half the amount necessary, adding equally to each gore (Figs. 91C, 92A).