Chery,
The easiest way is to measure your window to determine how wide you need it to be.The length is usually a third of the window,or what ever length you'd like.For the width you want it to be one and a half or double your measurement.Then you can go to the fabric store and buy the pleater tape,and hooks.This tape you sew on flat to your piece of fabric,then when you put the hooks on it forms the pleats.This makes it very easy,and also when you need to wash the valance,just pull out the hooks,wash,iron and re-hang.HTH.
Kathi
PS,You might want to go to your local library and check out "Singers,sewing for the home",and/or "Singers more sewing for the home".Both are excellent books.But if you'd rather buy them,they are available at most fabric stores.

Thanks, Kathi. I had not idea there was something called 'pleater tape.' Almost sounds like drapery pleats. Are they the same? chery

Yup they look just like ready mades when you are done.Good luck,let us know how it turns out. Make sure you tell the clerks you want pleater tape for draperies.
Kathi