Stomata are pores on the epidermis of the plants and they allow oxigen to get in and carbon dyoxide to get out, water can also leave the inside of the plant from here. These ones are from the epidermis of Crassula ovata. We can see how the epidermical cells are colorless but the stomata are green because of the chloropasts in them.
To observe this with your foldscope you need to take a leaf of a succulent plant and you have to take off the thinest layer the lief has on its surface (this is the epidermis). Put this layer on the microscope and that’s it. All plants have an epidermis, but it’s easier to obtain from a succulent.