Don't try to devise a system of your own.
Having decided on your system of classification, begin to classify.
This is one of the many things which can only be learned by doing.
Give fiction no class number, but an author number or "book-mark" only, as explained in a later chapter.

Give all biography a single letter as its class number, and follow this by the author number.
Distinguish all juvenile books, whether fiction or other, by writing before their numbers some distinguishing symbol.
Take up first, in classification proper, the subjects of history and travel, which will be found comparatively easy.
It is easier to classify 25 or 50 books at a time in any given class than it is to classify them singly as you come to them in the midst of books of other classes.

Consequently, group your books roughly into classes before you begin work on them.
As soon as a book is classified enter it at once in your shelf-list--explained in a later chapter--and see that an author-card for it is put in the author catalog--explained later--with its proper number thereon.
If, after you have made up your mind, from an examination of the title-page, or table of contents, or a few pages here and there, what subject a book treats of in the main, you are still in doubt in what class to place it, consider what kind of readers will be likely to ask for it, and in what class they will be likely to look for it, and put it into that class.

In doubtful cases the catalogs of other libraries are often good guides.
Keep your classification as consistent as possible.

Before putting a book, about which there is any opportunity for choice, in the class you have selected for it, examine your shelf-list and see that the books already there are of like nature with it.
Classify as well as you can, and don't worry if you find you have made errors.