The simplest streptohedron is a Sphericon. Imagine a CUBE of wood mounted on diagonal corners and turned to Back To Back Cones. Now if the cube is actually made up of TWO pieces of wood - with a clued paper joint in the right place, you can part the back to back cones And if you turn the bottom half 90 degrees and glue the pieces back together - you have a Sphericon.
Maybe these illustatrations will make things clearer. And if you click on the image, you can download the SketchUp file of these virtual 3D objects and, with SketchUp or SketchUp Viewer, get to know and understand how this Streptohedron idea works.