Place a drop of water onto the camera lens. Make it as spherical as possible.
Position the object you want to image over the top and adjust height to focus.
Take the image.
For other easy ideas to turn your phone into a microscope, why not come to the Ashmolean museum in Oxford, UK. just one of the many events going on as part of the Oxford Brookes university 150th anniversary celebrations. http://www.brookes.ac.uk/150-years/live-friday/