Measure the width of the window according to where you install the curtain rod, such as inside the window casing or outside the casing on the wall. Double this measurement for calculating yardage. For example, if the measurement is 45 inches you'll need 90 inches of material to cover the space adequately.
Measure the length you want the curtains to be and add 4 1/2 inches to the measurement. If the curtain needs to be 60 inches long, you will need 64 1/2 inches of material.
Purchase your yardage. Using the previous example, you would need 2 1/2 yards by 2 yards of material (rounding up). However, the exact yardage you purchase depends on the width of the fabric you select. Bolt width varies greatly. If the fabric is 90 inches wide, you would buy 2 yards of fabric. If the bolt is only 45 inches wide, you'll need 4 yards of material.
Purchase the same amount of plain white flannel for the lining.
Wash, dry and iron the fabrics.
Use your original measurements to cut the fabric and lining into sections for each curtain panel.
Trim 1 inch off the width of each panel of lining and 3 inches off the length.
Press a 1/4-inch fold along each vertical edge of the curtain fabric. Fold toward the wrong side of the fabric. Repeat the process with a 1-inch fold.
Press a 1/4-inch fold at the top and bottom of the curtain fabric. Repeat the process with a 2-inch fold.
Position a section of lining in the center of one section of fabric, placing it on the wrong side. Fold the edges over to catch the lining and pin it in place.
Sew the edges with a standard 3/8-inch seam. Trim off excess threads as you finish each seam.
Repeat the process with the top and bottom hems.
Sew a straight seam 1/2 inch down from the top of the curtain to create a pocket for a curtain rod.