On most curtains & window treatments, it takes two to three times more material/curtain to provide the proper fullness in appearance when installed/hanging on rod than the actual window width.
Measure the width of the rod from left to right. A common rule for displaying curtains properly says the curtains finished width should be at least 2 times the width of your window (if not more - sheers can be 3 times the window width) to achieve a look of proper fullness.
Example: If your window measures 36" wide (window width x 2 = 72"), you need curtains that will give a minimum width of 72" or 2 panels for that window. In this case 2 panels will give about 100" to 120" of width which will look nice and full. Always round up to the next full number.
Common Fullness multipliers (multiply by your measured width ):
Measure the drop of the window or area you want to cover from top to bottom. Overall curtain drop, anywhere from the sill to the apron or to the floor is a personal preference and need not be exact. By moving your rod up or down a little you can adjust the position of the curtain at your window.
If sill drop is desired, just below the bottom of the sill is correct. Other hanging styles bring the curtain to the floor in order to “puddle".
For a maximum room darkening effect, ensure that your curtain extends a reasonable amount around the window frame in all directions.