I simply use a wash cloth or hand towel. Simply dampen one end, wipe your device, use the dry end to dry and polish - easy-peasy.
What you need to be careful of:
1). Always use a CLEAN cloth. So if you're out and about and decide to use the tail of your t-shirt be aware that small particles of sand/dirt might be on your shirt and scratch your device,
2). Don't use any chemicals like Windex etc. Not good for the coating. I did get a can of Zagg screen cleaner (it was a freebie sent with some other items) and it was really good. If you decide to use a cleaner get one designed for screens, not windows.
3). ALWAYS USE A CLEAN CLOTH (worth noting twice because this is the most important part regardless of the liquid you use).
This question comes up a lot. I'm sure mostly from young men who have no experience cleaning 'stuff'. Take it from an old woman who knows about cleaning and tech. Don't over think it. Don't make it expensive when it doesn't have to be. But DO USE A CLEAN CLOTH. 😀