Dewey Decimal help

I've recently joined the Library Committee at our Quaker meeting. The library was well-maintained in the past but during the pandemic nothing happened so there is almost no one on the committee who has experience. I have five cases of boxes that were donated during the three-year lull (in my dining room!). I want to put Dewey numbers on them but I'm totally intimidated. I do understand that church libraries have different rules because otherwise everything would be categorized under the name of our religion. Can someone refer me to a basic Dewey Decimal tutorial for small church libraries?

If you enter the books using library sources like Overcat, most of them will end up with Dewey numbers.