Measure the length of the window and determine where you want your drapes to hit -- the window sill, just below the sill or to the floor. If you choose floor length then you will want to decide if you want the drapes to fold onto the floor or if you just want them to brush the floor.
Measure the width of the window to determine the size of the panels. According to potterybarn.com if you want a tailored look to the panels, which means the panels will meet precisely with no additional fabric, then multiply the width of the window by one. For a standard look to the panels, this means that there is slightly more fabric than necessary, then multiply the width by one and a half. Obtain a full look, which means the panels have a lot of extra fabric to give it more of a plush look, multiply the width by three.
Hang the curtain rod by measuring the width of the window and adding an additional 1 inch to 3 inches on both sides of the window frame. The added inches will allow the drapes to be pushed to the side when they are open. If you plan to use finials, which are end capes for the rods, or hold-backs, which attach to the wall next to the window to hold the drapes when open, then you will need additional room on either side of the window.
Measure up 4 inches from the trim to hang the rod. Create the illusion of a taller room, by hanging the rod closer to the ceiling. Always remember that the higher the rod, the higher the drape will be from the desired ending point.
Mount the rod to the wall. Use your measurements, the included rod hardware and appropriate tools. Insert the drapes onto the rods using the pole pocket, clips or round rings, purchased separately.