Have your multi-sized pattern, a ruler and a pen to hand. Then, place the ruler against the pattern in order to connect the two last sizes on one corner of a pattern piece.
Draw a line extending to the outside and repeat this step on all the pattern pieces corners and markings.
Then, measure along the lines the distance between the two sizes and report this measurement on the extended part of the line. Be sure to do this on all the pattern corners and markings. If needed, multiply the measurements to add more than one size.
Finally, draw the new outline using the reported measurements on the extended lines.
When drawing curved edges, use the existing lines as a guide to create a cohesive curve.
Once finished, be sure to test your new pattern first. Get familiar with the designer standard for height and bust cups and adjust if needed to preserve the proportions of the garment.