- water (water from boiling pasta or potatoes is best as it has some starch in it already, but plain tap water works fine too)
- cornstarch or wheat flour or rice flour (oat flour would likely work but I've never used it...)
A couple of important note: 1) always press the fabric after starching before cutting your pieces. The iron should be a little cooler than normal so you don't scorch the starch. 2) let the starch soak into the fabric fibers before pressing -- that way its not just a flaky layer on the surface that comes of as you handle it... 3) insects love starch. Don't dunk everything in your stash and then store for months! I haven't had problems when I've cut starched fabric and used it in a block but if you're in a damp, bug prone place beware... and 4) remix up any homemade starch in a bottle before spraying -- it can separate if stored for some time... and it can mold if stored too long too -- its a food you're using...
If you want really stiff fabric make the starch really thick (clumpy thick gravy) and dunk the fabric into it. Squish the excess off the fabric (Don't wring it or you get lots of wrinkles and are hard on the straight of grain). Lay flat to dry.
If you want stiff, but not cardboard make the starch a little runnier. It can be sprayed onto the fabric with a plant spray bottle.