Sorry about the slow response, this is a much discussed topic, with many dimensions.
Assuming we are only considering external surfaces, the first priority is to do no harm.
More optics are ruined by zealous cleaning than ever were hurt by dirt.
The recipe is never scrub and go the extra mile or two to avoid scratches.
So step one for cleaning optics is to turn the lens upside down and blow off the dust with a puffer, ( usually a Rocket Launcher) and then to rinse the surface under clean running water. (This is where waterproof gear really makes a difference).
After that, if needed because of stuck on grime, carefully rinse with soap and water on your finger tips. In my experience, licking the lens clean with your tongue is usually quite effective, the saliva cuts the grease involved and the lens can then be easily rinsed clean.
After that, tap dry the surface with a clean cotton cloth (super old well washed T shirts are ideal).
Never wipe or scrub. Hope this helps.
As above, but a good quality microfibre cleaning cloth such as the Baader Optical Wonder cloth, is as good if not better than a clean cotton cloth. Remember also to clean only when absolutely necessary to further minimise the chances of damage. A rocket blower is a must to get rid of potentially abrasive dust particles and a good goat hair lens brush may also be useful. Be especially careful after birding on the coast when exposure to spray can leave harmful salt on lenses. Careful rinsing with of the lens surfaces with water as described above is the best first step here.