Knee breeches were worn in colonial America and in revolutionary France, so you will need many pairs to costume either "Les Miserables," "1776" or any school pageant involving the Founding Fathers. You can convert an existing pair of trousers into knee breeches or alter a trousers pattern to make them. Most male costumes from this era will include a long waistcoat which covers the waistband of the trousers, so you can make a very easy pair using a pajama pattern.
Try on the slacks and place a pin in the side seam 1 1/2 inches below your knee. Measure this distance up from the floor. Mark this distance up from the floor on the other side seam and both inseams.
Take off the slacks and draw an line around the legs of the trousers connecting the pins using a fabric marker.
Cut off the legs of the trousers along this line.
Finish the cut edge with an overcast or zigzag stitch. Iron up a 1-inch casing on both trousers legs.
Sew the casing by stitching a straight stitch 1/4 inch from the turned up edge. Leave a 2-inch opening for inserting the elastic.
Cut two pieces of 1/2-inch-wide elastic equal to the distance around the top of the wearer's calf plus one inch. Thread the elastic through the casings using a large safety pin. Overlap the ends of the elastic 1 inch and sew them together. Use a wide zigzag stitch down the middle of the overlap. Stitch the openings in the casings closed with a straight stitch.
Measure from the wearer's waist to a point 1 1/2 inches below his knee.
Measure this distance down from the waistline on both the front and back pieces of the pattern. Fold the pattern at these points so that the fold is parallel to the waistline. You may cut along the fold or, if you wish to preserve the pattern for later use, use the folded pattern to you cut the fabric pieces.
Cut the fabric with the altered pattern and make up the trousers, except for the hem.
Make a casing as described in the first section, at the bottom of the pant legs.
Choose a pattern with slim legs.