Patents are designed to protect your right to exploit your invention for financial gain, provided the law recognizes you as the first person to have created your invention. In many countries of the world, the law recognizes an inventor as the first person who files a patent for a particular invention. So in these countries, claiming the right to your invention is a matter of making a ptent filing as quickly as possible once you have worked out your invention. However, in the United States, the patent system recognizes the legitimate inventor as the person who first invents the invention, not the person who first files the patent for an invention. This is the reason why it is important to keep an inventor's logbook.

You can either buy a special inventors logbook, or use any other way you can think of to securely record the progress of your invention efforts. The purpose of the inventors logbook is simply to be an irrefutable diary of your progress, so that if it comes to a legal conflict, you can produce your logbook and some supporting evidence to substantiate your claim that you were the first to start working on your invention, and the first to "make a working model" of your invention. So this means that you should use a logbook that is tamper-proof, and get someone to witness your entries in your logbook. For example, don't use a legal pad or ring binder for a logbook - it's easy for an adversary to claim that you deliberately added or removed pages from your logbook to bolster your case.

You should start keeping your logbook as soon as you conceive of your invention, even before you start working on it. But remember that patents only recognize workable inventions, not pipe dreams (you can't patent a Star Trek transporter if you can't explain how to construct one). So the real important part of your logbook is the part where you record your progress in trying to make your invention work. In other words, how you "reduce your invention to practice".