Swags and tails are a classic curtain top treatment that consist of soft folds of fabric draped along the width of the window top, with gathered or pleated side panels. Your window must be fitted with a cornice or a mounting board.
To estimate the size of the swag you wish to make, use a tape measure, holding it at each top corner of the window and allowing it to dip in the middle by about 15 to 20 inches. This length will help you calculate the width of decorator and lining fabric needed for the swag. To calculate the length of fabric for the tails, measure the length at which you want the lowest and highest points to fall. The width of the tail pieces should be about three times as wide as you want them to be when draped or pleated. Test your dimensions by cutting a piece of muslin and draping it around the window. This will enable you to make any necessary adjustments to your calculated dimensions before cutting the decorator fabric.
Your pattern piece of fabric for the swag will be about the width of the window along the top, but wider at the bottom, with diagonally sloping sides. You need to mark the fabric at even intervals to indicate where you will make the folds for the draping effect. Check the pattern piece by pinning it at the folds onto your ironing board, and make adjustments to the folds as needed. Trim the excess muslin, but remember to allow for seams, then use this as a pattern for cutting the decorator and lining fabrics. Sew the outer fabric and lining with right sides together along the top and bottom edges, then turn the swag so the right sides are facing out. Pin the pleats in place, then stitch across all the folds of the gathered fabric at each end. The swag is mounted first, and the raw edges of the sides will be covered by the tails, which are mounted over them.
The pattern pieces for tails should be about three times as wide as you want the finished width to be. Start with a rectangle of fabric with the longest side about the length you want the lowest part of the tails to be. On the other side of the rectangle, mark the point where you want the highest part of the tails to be, then draw a straight line from this point to the opposite lower corner to form a rhombus shape. Line the tails by placing the outer fabric and lining with right sides together. Sew the side and bottom seams, then turn the piece right sides out. Make concertina-style folds, starting from the outer edge, and press the folds well. The tails for the other side of the window will be a mirror image of the first one. Mount both swag and tails onto the mounting board with a staple gun or upholstery tacks.