Myself and a colleague have built and used two Foldscopes thus far and are very impressed. We tested it with both a prepared glass slide and the paper slide making materials provided in the kit.
One thing we noticed was that a glass slide, being fairly heavy in relation to the scope, is hard to keep in place while viewing. So we came up with the simple solution of clamping the slide in place with a binder clip (see photo). Try it!