Here's what Apple provides on this topic. Unfortunately, it doesn't provide much about how to best clean the keyboard's keys.
Apple wireless or USB keyboards, mice, and trackpads
First, power off the device. If your device uses removable batteries, remove the batteries. (If your device is wired, simply unplug it from your computer.)
Next, clean the outside of your device with a lint-free cloth lightly moistened with water. Don't get moisture in any openings or use aerosol sprays, solvents, abrasives, or cleaners containing hydrogen peroxide.
Ref: How to clean your Apple products - Apple Support