|I don't know if everyone but me already knows this, but I figured it out recently and it has been very useful in making tailored and fitted clothing.
This method is helpful when altering or fitting patterns or RTW in fabric that is suited to the use of wax transfer paper (tracing paper).
Step 1 - Adjust one side as needed - sleeve or side-seam, bust dart, pant leg, and so on. Press the area flat. I recommend that trimming be done after the both sides are altered.
Step 2 - Align the altered area so that it lays directly on top of the other side, aligning the ORIGINAL seam lines. Pin or use bobby pins to hold the layers in place. For example, the modified bust dart might lay above the other, with the folded edges aligned. If you cut carefully at the beginning, you can align the raw edges at this point.
Step 3 - Place wax transfer paper underneath the bottom of the second piece. This makes a 'sandwich' that allows you to see your new stitching and also that the two garment areas are aligned properly.
corresponding unaltered piece
Step 4 - With a tracing wheel, trace over the NEW stitching line. Considerable pressure may be needed, depending on the thickness of the fabric.
Step 5 - Use the marked line as the new stitching line to complete the alteration.
When the alterations are complete, the new line can be transferred to the pattern tissue in the same way, substituting the tissue for the "corresponding unaltered piece" in the sandwich in Step 3.
Happy sewing everybody!