You will need a front cover, some material for inside – this can be old blankets, fleece material or any other thick material, and then the insulated backing material.
The interlining should measure the width and the height of the window + frame. Place on back of front material, turn a 10 cm hem and sew around the 2 sides and the bottom. (this can be hand sewn with locking-in stitch if you don’t want the stitching to show).
The backing material be the same size as the front material. Turn the 10 cm around the sides and the bottom and sew.