"There could be a hundred paintings
in every one painting,
depending on when you stop."
~ Peter Doig
When you are new to art journaling, consider jotting simple notes to remind yourself how you created each page. There are many ways to do this, but the easiest is to make notes on the back of the page. In bound journals/altered books, you can save a page toward the end and make notes about each page. Many artists document the process in their blogs. You can add tags or a comment in your photo editing software or place notes in a little envelope taped to the page. Lots of options.
For altered books or bound art journal pages, I find that blogging is easiest for me. About 50% of my art journaling is on loose 9x12" watercolor paper (i.e., Art Journal: Collection) so I write cryptic notes on the back. It's especially useful when trying new things or playing around with the order I do things. I like to write the order I do things, the art materials used, whether there was a method to my madness, etc.
After you art journal for a length of time, you will likely be able to look at your work and know what you did and what art materials were used. You'll be able to tell the difference between acrylics and watercolors, microns and pitt pens, etc. I'm such an analytical artist, so I don't know if it's just my thing. I love the concept of documentation. What type of art documentation works for you? Or is it not a priority? Tedious?
Here's more in the step-by-step art journaling series.