How do I blend between two pattern sizes?

Blending — or grading — between sizes is how you fit a body that does not match one column of a chart. Almost nobody does.

Trace the pattern first. Do not cut the original. Tracing costs a sheet of paper and means the pattern survives the next change.

Mark the points. At each key location, mark the line for the size you need there: bust on one size, waist on another, hip on a third.

Join them with a smooth curve. Use a curved ruler laid over the marks, sliding it until the printed curve matches the line you are continuing. Freehand joins look fine on paper and read as a lump in the finished garment — the eye is extremely good at spotting an uneven side seam or armhole.

Blend gradually. Spread the transition over as much length as you have. A blend that happens over 3 cm creates a visible kink; the same blend over 15 cm is invisible.

Do the same on both pieces. Front and back must be blended identically or the side seams will not match in length. Walk the pieces against each other afterwards and check.

Then check the seam lengths, not just the shapes. Two pieces can look right and still differ by a centimetre, and you will find out when you sew them.