I have done this since July of 2003 when I set up a 20 gallon with two babies in it. They are now huge and in a 50 gallon.
I keep the notebook in a drawer next to my tank and keep track of water testing, water changes, anything new I add to the tank and sometimes make quick notes on fish behavior.
It really doesn't take long and I can look in it to find out when I did certain things or used medications.
Just a simple notebook and a pen or pencil is fine. When I first joined this site, someone suggested it but I don't remember who.
You can see that the pages are a bit wrinkly since I have spilled water on the notebook.