[header] How to document your invention [title] Buy a bound notebook. [step] Despite technological advances, the best way to document your invention remains hand-writing details regarding your idea, descriptions, and tests on paper in a bound notebook. [substeps] Look for something with a hard cover, and pages that are sewn in. 
This makes it more difficult for a later challenger to argue that you added something after the fact. Some of these books have blank pages, while others include lines or grids.