The sewing instructions have been lost for many years, this is what I do. Serge the leg bottoms and the top waist band before sewing. Sew the front and back side seams together, then serge the seam. Next sew the the inseams and serge.
Turn one leg right side out and insert into the other leg to sew and serge the centre seam front to back. Next turn over one and one quarter inches on the waistband to make the casing. Insert a tag of grosgrain ribbon to permanently mark the back. Sew around bottom edge of the casing leaving a two inch opening. After threading the elastic and sewing the ends together, sew the casing closed. Move the flannel around the waist band to make the gathers even. Then while holding the fabric with the elastic stretched to completely smooth out the waist band, top stitch from the top edge and in the middle to keep the elastic in place.
Now for the big question, what should I make for a Halloween costume party on October 26th?