Keeping a garden journal, is so useful, I can"t even tell you! I had started mine many many moons ago, and really should start to update it again, because since then, i have spent 5 years adding more plants to my land that i even care to count. I find myself going back to this journal in search if info I had marked down on a specific plant, that i had long forgotten about.
Your journal doesn't have to be fancy smancy, mine is just an old document folder, with paper in it and a few pockets. You can add or remove papers as you see fit. Kinda like a garden grimoire.
In my journal I have pictures of specific plants in my garden, and on the next page a few pieces of paper with the information i have observed about this plant growing in my garden, such as, when it blooms in my garden, how much sun it gets in a particular spot, and if I moved it, i mark down why i did, (maybe it wasn't getting enough sun or moisture in a certain spot, etc), where I got it from or who, if it can be divided or.....you get the idea!
I will rip out any inspirational pages from garden books, that I like. i might use the idea, or run with that idea into something else for my garden. Maybe I just like the way they planted their hostas...whatever...anything. Like that rickety ole arbor made from fallen branches, rip it out and save it for your journal, now you have a way to go back to it, if you decide you want to make one for your yard!
I also like to save the plant markers, that come with plants I have purchased. Just a handy dandy thing to have when your plant doesn't survive the winter and you cant remember the name of it!
So get those creative juices flowing and start that garden journal!