I went to Staples and got a 240 page 5"x7" journal with a dotted grid. It’s got paper that is thick enough that nothing bleeds through. It has a couple of ribbons for marking a place and a pocket with a little ruler that has templates with a few small shapes that I never use. I tried using a small three ring binder, but realized it was too awkward to carry around and I realized that trying to put certain things in different sections actually was working less well than just using an index. The guy who invented bullet journals also sells one with a few pages that give the basics of the system. I use my right hand margin to record hours - if there’s just one separate contract I’d just circle those hours. If there’s more than one I’d use a letter to keep track of them. When I’ve charge my clients, I cross out their hours.
You can definitely learn the system by just reading the book and watching a few youtube tutorials.