Hello Grace! Here are two sites that will give you some advice! You can also buy a sewing pattern for scalloped curtains at your local sewing/craft store like JoAnn Fabrics, Hobby Lobby. I do find that buying the pattern and the material to be more expensive than just buying the curtains. If you can find the curtains on sale it's even a better deal! JC Penney online often has curtains for a good price.
First go buy you pattern for Swags and Tails from Fabric shop. Do talk to the assistants, they should be knowledgeable, either help you themselves (If you go on a quiet day) or point you in the right direction. My advice would be to try it out in a small way, before spending a lot of money... Maybe practice on an old sheet?
You may find it cheaper to purchase ready made the size you want.
Hi Grace....You can buy a pattern for a scallop swag curtain at Joanne Fabrics. They have all different styles and in one pattern you can get sometimes 5 different patterns. Sometimes those patterns can be anywhere from $12-20. If you are pretty knowledgeable with sewing though you can make your own pattern. I would always play with a plastic table cloth(dollar tree) and because it flows like regular fabric I would mark my pleats with a sharpie, pin the pleats up and play with the tablecloth on the window. I made some amazing patterns doing that. Swags and jabots are pretty easy so if you can sew clothes, you have this covered:) If you are unsure, buy a pattern. Follow the size of the window on the back and it will tell you how much fabric you will need. You will also need a board cut (the length of you window ) X around 6/7 inches in width. Once you have your pieces of swag and jabots....you will need to staple the top of those sewn pieces to the top of the board. They should have these directions on the back of the pattern as well but this gets the curtain assembled on the board. You will want L shaped angle brackets to screw to the wall and the board over the window. Good luck! Here is one you tube video for you: This is a scalloped valance: