Atmospheric methane is the second most important anthropogenic greenhouse gas after carbon dioxide. The present atmospheric burden of methane is unprecedented in the last 800,000 years and has almost tripled since the onset of the industrial revolution. Arising from an interregional technical cooperation project aiming to assist countries in building capacity in stable isotopes analysis of atmospheric greenhouse gases and accurately determine their source, this publication provides high-level guidance to laboratories that are new to the use of optical instruments to measure methane and its stable carbon isotope ratios. The publication covers analytical infrastructure, sampling strategies, as well as techniques for measurement, calibration, data processing analysis and interpretation. Further emphasis is on technical solutions that ensure that observations are accurate and comparable, as well as on sustainable data management techniques, and quality control procedures, to maximize the impact of newly generated data.