Tracking your bonsai is one of those things that we always intend to do.  I know I have had all sorts of iterations. I started this last year.  It can be done on your phone - and I say whenever you are doing work on a tree, you should take your phone with you and do this.

Install OneNote on your phone.  It is available for both iPhone and Android.

Create a Notebook and give it a name you like.

Open the Notebook and Create a New Section - give it the name that allows you to know the tree.  The Sections page will look like this:

 If you have significant notes to make, you can do that on your PC.  Android phones support creating a shortcut on one of your home screens that points directly to the Notebook.  Since you can do much or all of the work on your phone as you work on the tree, you don't have to rely on memory or handwritten notes.  And you can add pictures as the tree progresses.  

Article by Greg Filter