I followed all the steps in this tutorial.....
http://made2style.com/2012/02/22/diy-roman-shades/ .....the only difference was that I used Mod Podge because I have a lot of it and I know it works well. My mini-blinds were actually a bit narrower than my window openings, but I still measured for the window opening and centered the mini-blinds on the fabric and the hardware to my window frame. The fabric just skims the framing so the sun doesn't come in past the fabric on the sides.
These two happen to be in the kitchen and the late afternoon sun shines in to the table when we're trying to eat. Not much fun. Otherwise, they are kept up. I've done the same thing in our bedroom, one spare bedroom and I'm using this same fabric in my sewing room. I still have a couple of rooms to go, but haven't decided on the fabric yet. A med-weight upholstery fabric works best and you could line it with a lightweight lining, but I wouldn't go too heavy.