Your Rikenon works like a Pentax-K or Pentax-M lens.
- Change camera menu setting to allow for manual aperture ring
- Move the aperture ring off the "P" setting and use it to set the desire aperture
- Use M-mode and the green button to meter the appropriate shutter speed
- Retain these readings until either the light changes or the subject changes or you wish to use a different aperture
Your lens, like all Rikenon P lenses has the dreaded "Ricoh Pin". There are two types of Ricoh pin. The dangerous one is a spring-loaded metal pin. The benign one is a spring-loaded ball. If you can remove the lens, yours is probably the benign type. Double-check to be sure. If it is the pin type, don't mount the lens on your camera until you have it removed.
I own the Rikenon P 50/2 and it is a decent lens. Despite the plastic build, the controls are smooth and easy to work and as you may have noticed, it is tiny. Optically, it is very sharp.
Steve