Cut the sides of the old undies open and lay them flat (and please don't judge me on these very old Tommy Hilfiger panties)
This is the most important part! Over the tracing paper, carefully lay the fabric flat. The elastic edging will be stretched, but the fabric itself shouldn't be stretched - this is how to make sure the shape stays exactly as needed.
Use a ruler or small weights to hold it in place while tracing. Then, while laid flat, but not stretched, measure the elastic seam of the front, back and side (leg hole) and note for when we cut the FOE.
Cut out your pattern piece and mark a line where the seam of the back & front are sewn together and mark the placement of the lining fabric.
Now cut out a lining piece using the pattern and either by folding (or cutting the pattern in half at the seam line you've drawn), cut out the front and back piece but add an extra 1/2 inch on both pieces at the sides and at the line where the pieces will join (for the seam allowances). Transfer placement markings for the lining, so you end up with 3 pieces.
With right sides facing, sew or serge together the front & back. If straight sewing, I recommend doing a second line of stitches just next to the first for added strength.
Pin the lining to the wrong side of the undies lining up on markings (this should cover the seam).
Now cut out the FOE for the front, back and both legs using your measurements. Sew on the FOE to the front and back with a wide triple zig-zag stitch (5.0 wide by 2.0 long).
The trick here is to slightly stretch the FOE while sewing. It's easiest to pin the top and bottom ends of the FOE in place and then stretch the FOE as you sew to make sure there's the perfect amount of give and still fit just right. DO NOT stretch your fabric - this will affect the fit.
Repeat with the leg hole areas.
With right sides facing, stitch or serge together the front & back at the sides. Again, if stitching, do two rows for added strength.
Trim off the excess fabric on the sides and your new perfect fit undies are all ready to wear!