If you are using source control or making copies of the file at various steps along the way, it is important for your log file to remind you of what you did between each of the break points. For instance:
Joined the lines; saved the result as dictionary-joined.db
Changed the marker \g to \ps; saved result as dictionary-mod1.db
etc.
As you write in your logfile, think about what kinds of things you will wish you could remember if you need to look back at what you did. You may not need to know everything you did, but if you have made different versions of the file or different folders, it will be important to mention the file or folder names explicitly in your log, and try to indicate what the difference between them is. For instance: "Edited spanish-dict-mod03.sfm to remove empty fields; saved result as spanish-dict-mod04.sfm".
Decide if your logfile will be in these working folders. If so, make sure you close it in your editor when you switch to a new folder, and be sure to open the copy in the new folder. Alternatively, you may decide to have only one copy of the logfile and keep it at the top level.
It's easy for your logfile to get confusing when you start down one path and then back up and start over. Develop an approach for when that happens.