For width, go a little more than half as wide as your curtain rod for each panel. Length is up to you, but be sure to add about two to six inches on the bottom for a finished hem and two to four inches on top for the rod (about four to ten inches overall).
Snip the fabric you’ve chosen to the desired length and width. Since the width is pretty flexible, it’s easier to simply leave it the width of the fabric rather than cutting, as long as it’s in a relatively close range. For the top and bottom, don’t worry too much about cutting a perfectly straight line -- you’ll be folding over both.
Place your fusible web tape (you can find this at craft stores -- it’s the stuff that lets you iron fabric pieces together) roughly two to four inches down from the top on the back side of your fabric. Use a measuring tape and pencil to be sure you’re getting a straight line. Remember, you’re doing this to create the pocket that your curtain rod will go in, so make sure to look at your curtain rod and take its size into account. Then, just fold over the fabric from the top and iron from one side to the other to get a seam with no creases. Do the same thing at the bottom of your fabric!
Now learn how to fake expensive curtain hardware!