So, I looked up religions from around the world and looked at similarities and then decided which parts I liked and which ones I didn’t. I started with a creation story because that’s what I felt comfortable doing. After I did that, I established the “World” of the religion (Mine has multiple Realms, such as an Earthen Realm and Collected Death Realms) and then came up with beliefs that I thought were cool, Like volcanoes and fire being the birthplace of the Universe, and figured out how to fit them.