I use a short steam box for bending ribs for kayaks. The ribs are 1/4 inch thick and 3/4 inches wide and usually less than two foot long. I use a long steam box for steaming coamings that go around the cockpit of the kayak. The wood for the coamings is about 7 feet long and 3/4 inches by 1-1/2 inches in cross section.
Step 1: Materials list
You will need a coat hanger or a few feet of heavy wire as supports inside the steam box to keep the wood off the bottom so heat can get to it from all sides.
You will need a cooking pot. A two quart size is good for starters. You can fill it two thirds full and generate steam for several hours.
You will need a heat source. If you plan on working outdoors, a camping stove works fine. If you work indoors, you can use a hotplate. The hotplate I use runs 750 watts. That is hot enough for the three foot steambox I use and just hot enough for the 8 foot steam box.
Since first writing this Instructable, I have gotten my hands on a wallpaper steamer. It works much better than boiling water on a hotplate, mostly because it runs at a higher wattage and puts out more heat. If you can get a used one, go for it.