What is the best way to stiffen fabric for making roller blinds?

I've made shades with fabric. I used a kit; this isn't neccesary. You just have to supply the roller. A liquid fabric stiffener would make the fabric too stiff to roll. Go to a fabric store and buy the heaviest weight iron on interfacing. Just iron it on the back of your shade fabric, leave an inch if you plan on lining.To cover the interfacing, get an inexpensive lightweight fabric e.g. muslin, fold under the fabric and lining one inch and either sew on edge or use a fusible hem product. Good luck! Sherpy