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.
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.
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.