A Magickal Journal is not too different from an ordinary journal or diary. You should keep track of any workings you have performed but should also include other details. Note any activity or exercise you had done that day as well as diet. If you created or worked on any artwork make note. You should also note your feelings and moods throughout the day. Also track if you have experienced results from prior workings. If you are experimenting with belief systems, note what system you were using or performing that day.
If you are already in the practice of keeping a journal, you simply need to add these elements to your current journaling if not already present.
As SOMa is evidence based, keeping track of workings and your results is paramount. If you are not getting results it is an indication either you are doing something wrong (less likely) or magick isn't working for you (more likely). If, after a period of several months you are consistently not seeing results it is an indication you should stop, reassess, and perhaps move on. Magick is not for everyone, and results will vary day to day, week to week, month to month, and most importantly person to person.
The SOMa system involves deliberately inducing changes to your brain and mind. If you are not seeing these changes, the system is not working for you - you are not failing the system.