Hi. Here are some good directions. Hope these help...
Use a mild soap mixture - a gallon of water to 1/4 cup soap at the most. Use a soft sponge, wrung out well. It should just be damp, and wipe the wallpaper.
This video recommends a simple microfiber cloth: https://www.youtube.com/watch?v=zJD8w79FpJA
Wipe down with warm soapy water and a sponge. I would recommend blue Dawn.