According to a research performed by Japan Tourism Agency in 2015 (*1), about 56% of onsens reject the entry of people who have tattoos. These days, this is not always the case, and foreign visitors can be an exception. However, if you have tattoos, please make sure to ask in advance.
If you have tattoos, see the Tattoo Friendly Onsens page in our website. The onsens listed in this page are tattoo-friendly. Also, at the bottom of each article, you can find tags related to the article. The "Tattoo Friendly" tag indicates that the onsen is tattoo-friendly. We hope our website can help everyone find and enjoy onsens in Japan.
*1: http://www.mlit.go.jp/kankocho/topics05_000160.html (Japanese)
Here are the basic steps to take a Japanese bath. The steps may change depending on the facilities of an onsen.
Leonard Koren, Suehiro Maruo, "How to Take a Japanese Bath", Stone Bridge Press, 2006
Bathing experience in Japan may be different from that in you country, please make sure to follow the Japanese manners when you take a Japanese bath. If you don't follow, that would cause a disagreement with other bathers.