You could always book yourself onto a course...
Aeronautical Meteorological Observing - Met Office
When I did it, many years ago, it was one or two weeks at the college and then one week at LHR doing observations. The certificate sits in my cupboard. It's a skill you need to keep fresh. I sometimes make my own assessment and then look at the METAR, to try to retain some degree of ability.
Searching for 'met office observer handbook' will find the old guidance.
Mostly, these days, it's automated. Not always for the better, though a Cloud Base Recorder was most useful.