If the curtain will hang inside the window frame on a tension rod, measure the width (left to right) inside the window frame.

If the curtain will hang outside the window frame on a projection rod, measure the width outside including wood trim.  Also include “return” – the distance the rod projects outward at both ends.

Then take that measurement (the width) 1.5 times the width of the window.  Then take that total (in inches) and divide by 36, this should give you the number of yards you need to order. 

Ready-mades have a fixed width and length and come in valances, swags, tiers and panels.  Most are 60 inches wide and then vary in lengths.  All ready-made curtains will fit windows 27 inches up to 48 inches wide.  The amount of fullness will of course vary and is a personal preference.  Usually 1 ½ times up to 2 times fullness is recommended.