Get a copy of the subject list. You can get a partial copy of the list online from the Online Computer Library Center (OCLC), which owns the rights to the Dewey Decimal System. This free list contains almost a thousand subject classifications, which should be enough for small libraries and collections.
Assign one of the ten classes on the list to each book. Each class covers a major subject area, and has a specific range of numbers assigned to it. The classes are : Computer science, information and general works; philosophy and psychology; religion; social sciences; language; science; technology; arts and recreation; literature; history and geography. For example, a book about chemistry equipment is in the "science" class. All science books fall in the 500s range.
Choose one of the ten subject divisions of each class for each book. In the example, the book about chemistry equipment would fall under the "chemistry" division of the science class, which is the 540s range. If a book is about general science and does not fall into a specific division, just assign the book a call number of "500."
Look at the list to see if the book falls into a particular subject section of chemistry. A book about chemistry equipment falls into the subject section "Techniques, equipment materials," so we assign it a call number of "542."
Divide the subjects into even finer distinctions, if you wish. You will need one of the commercially available subject lists. If you do add any numbers, you will need to put a period after the first three digits and continue adding numbers after the period.
Add the first two or three letters of the author's last name after the call numbers, if you wish. This way if you have two books with the same call number, you can shelve them according to the authors' last names.