valve calibration

Good morning, afternoons or nights depending on the good country I would like someone to explain in detail how I can calibrate the valves of my car this is a Nissan 200sx Model 85 The engine is a CA20E, or I would thank you if you can send me the detailed diagram with images and theory how the blissful valves are calibrated, I would take it to a mechanic but I have no money and I would also like to do myself Much my email is see you soon