Adjusting the valves is not difficult, a big P.I.T.A. is getting to the top of the engine.
Then, there is not much room to move around and see what you are doing. I use an angled feeler gauge to get the angle straight with the lash gap for true measurement. Once I get the lash set and the set-nut tight I recheck the lash to confirm nothing moved.
These are low out-put, low compression engines so being slightly out of adjustment can be noticed, especially if the plug is slightly fowled and other things are not just right. (like a dirty air filter and old gas.)
The other problem with this subject is many members have reported taking their bike to the shop for valve adjustment and being charged for this work but the work was not done. Some have marked the valve cover cap with say a grease pencil, only to get the bike back and the grease pencil mark was not disturbed. (Some shops don't have experienced parts changers and get caught charging for service not performed.) Then the G.C. member decided to do the work them self to find the valve(s) needing adjusting after a trip to the shop.
A factory service manual is about 10 buck on tradebit, and will be a big help for the first timer.