1. Use a folding ruler or metal tape for measuring rather than a cloth tape which may stretch or sag.
2. Start the LENGTH measurement where you want the Rod hung (above the window), then measure towards the bottom of the window to where you want the curtains to end. This Length measurement is the FINISHED LENGTH of your curtain panels.
3. The combined WIDTH of both curtain panels (2 panels come in a set) should equal 1 and a half to 2 times the total width of your window (e.g. the width of your window is 60″, so your curtains panels combined should be minimum 90″-so you will need 60″ panels and you will achieve 120″ of coverage on you 60″ window and your curtains will be nice and full).
4. Start WIDTH measurement up to 4″ on either side of the window and follow through to other side. Use guidelines in tip #3 above to determine what your finished width should be.
5. You may even find that two panel widths is not enough coverage for the width of your window. If this is the case, you can purchase 2 sets and put the curtains all on the same rod.