If it's really dirty then I'll soak it when I wash my hands, dry my hands, and then use my shirt or a hand towel to dry off my watch. I also shower with it everyday . I've never been concerned about scratching screens with a shirt or towel as long as they're clean.
Have not washed, wiped, cleaned, polished with anything since my 42 SS arrived on 4/24. I wear it to the gym three or more times a week, mow the lawn with it, wear it every day and do not baby it in the least. Having done nothing to clean it, I am AMAZED at how clean and smudge free it looks. Apple did a magnificent job making this a low maintenance, every day device that requires almost no effort to keep in showroom shape.
If you must clean it, no secret to that. As others say above, water and a soft cloth. If gunk gets on hard to get places, a soft bristle tooth brush can be used, which also works great for getting at car wax that gets in cracks and crevasses when doing car cleaning.