Place the book in a class first. The 10 classes are 000-Computer science, information and general works, 100-Philosophy and psychology, 200-Religion, 300-Social science, 400-Language, 500-Science, 600-Technology, 700-Arts, 800-Literature, and 900-History and geography. For example, the book "Gray's Anatomy" is placed in the technology section. This is because applied sciences, such as medicine, go under technology and not science.
Put the book into a division under each class. Each division is two digits and a multiple of ten: 00, 10, 20, 30, 40, 50, 60, 70, 80 and 90. The unabridged Dewey Decimal Classification (DDC) index contains a complete list of all divisions. For "Gray's Anatomy" the division is the number 10 for medical sciences and medicine.
Place the book in a division's subsection. There are 10 sections under each division, resulting in 1000 sections used in the Dewey Decimal System. Each section is directly related to the division it falls under and has a single digit of one through nine assigned to it. As with the divisions, look to the DDC index for a list of subsections. Continuing with the "Gray's Anatomy" example, the book is about anatomy which has a "one" as its number. Now the book is placed under the Dewey Decimal Code of 611.
Add a decimal point and continue to classify further if necessary. Most libraries have the Dewey Decimal Classification Index with the complete list of classifications. A book could have a Dewey Decimal Code of 389.93042 and each number has a different classification. This can leave you with tens of thousands of possible classifications.
Go to your local library to use the DDC index and assign a Dewey Decimal code to a book. The index is not available online to the public or in a print form at any location other than a library.