Eagleson's Law: Any code of your own that you haven't looked at for six or more months, might as well have been written by someone else. (Eagleson is an optimist, the real number is more like three weeks.)

Writing documentation isn't so much a "how" problem, but a "why" problem. Most people writing code in their own time can't be bothered also writing documentation for it.

Writing code is fun (mostly). Writing documentation is not.