1. Use a metal measuring tape, not a ruler or cloth tape. 

2. Measure all windows, even if they appear to be the same size.

3. Note the location of each window by room name.

4. If inside mounting the curtain, make sure that the depth allows you to accommodate the board.  When taking the measurements, do not deduct for allowances.

5. Outside mounting allows the drapery to hang above and beyond window casing or molding.  It may also give the illusion of a taller or wider window.

6.When outside mounting, consider the depth of the blinds or shutters.  Make note of this return.

7. To determine width for an

 outside mount, add 2 to 3 inches to outside frame as a general rule.

8. For length, take into account the type of rod used; board, cafe, decorative pole, or pocket.

CHOSE A WIDTH:  A standard panel is 54 inches wide so if you desire more fullness, simply add to this number.  A simple rule of thumb; wider windows need more volume.  For valances, include the molding in this number needed always making sure that you include the outside molding of the window to determine width.  Also remember that the scale of fabric and print design play a role in determining both repeat and width.

CHOSE A LENGTH: Consider the height of your ceilings.  In older home (or those with 8 foot ceilings), mounting above the window trim works best.  In homes that allow more height, you might wish to add 4 to 6 inches above the window trim (to give the illusion of more space) or consider mounting just below crown molding for a more finished look.  Also determine if you want the length to touch the floors, trouser or pant break (add 4 to 6 inches) or puddle (add 12 to 15 inches).  Don't forget to include the rod choice in this measurement.  Tie backs, ruffles, and decorative fringe also effect the amount of fabric needed.

Consider using a professional designer or installer to come to your home and measure the windows before a yard of fabric is cut and sewn.  THIS HOLDS ESPECIALLY TRUE FOR CUSTOM CURTAINS.  Window treatment specialists keep your style of drapery in mind, noting all the architectural features that need to be accommodated such as doors, moldings, bookcases, and so forth.  The evaluate the curtain rod and support brackets as well and determine whether additional window hardware is required to carry the weight of the treatment.  They take exact measurements and check whether certain designs (such as cornices or valances) can be positioned properly.