Make sure to use the existing hem.
Turn the pant legs right sides together. You should see the ugly side of the hem.
Sew the inseams at 1/2 inch and finish the seam allowances. Press.
Turn the legs right side out and put right sides together. Stitch and finish the crotch. Press.
Attach FOE to inside of waist, stretching slightly.
Fold over waistband and stitch down.