Measurements:
1. Measure for your width. Our window was 35″ wide. We then had to determine how far past the window we wanted the boards to go. We added 4″ to each side of the width measurement. We took into consideration the width of the curtain rod and the fluff of the curtains. The rod went just past the frame.
2. Then for depth: Now, if you hang curtains under your board you need to figure for the rod. We used a 2″ curtain rod (it comes off the wall 2″). I had to consider the fluff of the curtains as well (this is to determine how deep to make the cornice). I wanted about 1″ clearance past the rod then I added 1″ for the curtains. So to add it all up 2″ rod + 1″ curtain + 1″ clearance past that = 4″ for the depth of my boards (for the returns, legs, or whatever you wanna call the side pieces).
3. Determine how long you want your boards to be. The standard seems to be to take the length of your window or window coverings in my case, divide that amount by 5 and take 1/5 as the length of your boards. My window coverings were 82″ so I came up with a little over 16″ and decided on 17″.