I use a 1:1 solution of Sta-Flo liquid laundry starch and water to heavily starch backing fabric and binding fabric (before I cut the binding strips). I "paint" the starch onto yardage until the fabric is saturated, toss in the dryer, then iron with steam. This leaves the fabric with stiffness similar to lightweight cardstock.
Using Sta-Flo allows you to control the degree of stiffness.
Many people also mix Sta-Flo and water to use as a spray starch, using a spray bottle.
Sta-Flo is sold in a blue plastic quart bottle and can be found in the laundry section of many grocery stores. (Not everyone carries it, though.)