If you want to add drapery lining, cut it to your desired finished dimensions. Center it on the back of the drapery fabric and pin together. Then when sewing the sides, top, and bottom, the drapery lining should be under the folded edges.
To determine the size to cut your fabric, take the dimensions you want your curtain panel to be and add 4” to the width and 12” to the height to account for the hems. To figure out what total width your curtains should be, multiply the width of the window by 1.5 or 2, depending on how full you want the curtains to look.
(Finished width) + 4″ by (Finished height) + 12″
This curtain will be 3 ft wide and 4 ft tall when finished so I’m cutting the fabric 40” wide by 60” high.