How to Keep a Meditation Journal
Keeping a meditation journal can be a really great practice, especially if you're just getting into meditation. There are a ton of benefits to doing this. It can deepen your mindfulness practice by building awareness and clarity about your experiences, and give you additional insight and understanding about what is happening within. It allows an opportunity for you to reflect, and connect experiences of the past, present and future. It can help you with goal setting around your practice. It can be helpful to have a history of your practice as a reminder of moving towards any goals you have. And from a mental health perspective, journalling, in general, helps us process emotions and stress and even trauma (always speak to a licensed mental health professional when using journalling for these purposes).

There are several different ways you can keep a journal.First, you can keep it short with just writing a few sentences, a paragraph or even just a couple of words about your experience in your daily practice. The alternative is to do longer entries with more information, allowing yourself to go deeper into aspects of your inner world. Either way, free writing is an option, or using specific prompts (ideas to come below). Two specific methods that I find beneficial (which can be brief or long, and free-written or prompted) are the "double entry" model where you write about your daily practice on the right hand pages of the journal and do weekly and/or monthly reflections about your practice on the left hand pages. The second option is to use the PIES method - this is especially beneficial if your mindfulness practice goes beyond meditation (i.e., informal practices, yoga, etc.). PIES stands for Physical, Intellectual, Emotional and Spiritual Bodies. For this method you divide your page into 4 quadrants, labelling them in this way and then reflect on how each body is feeling, what it needs that you can give it today, your limiting beliefs around that, and a self-forgiveness or self-compassionate statement about having those beliefs. This is the method I usually use.

image from: https://www.teacherspayteachers.com/Product/SEL-Journaling-Check-in-PIES-7180165
If you are unsure what exactly to write about, here are a few ideas:
The date and time of your practice, with the length and type of meditation practice, and perhaps an intention of your practice if you had one (I usually practice without a specific intention, unless I'm doing yoga nidra)
How you were feeling (mood, anxiety, etc.) before the meditation, emotions that came up for you during the meditation, and how you were feeling after the meditation
Anything unexpected that may have happened during the meditation as far as emotions, sensations, thoughts, images, memories, etc.
thoughts, especially commonly occurring/repeated ones, that came to mind during the meditation - including thoughts about journalling. Remember, when meditating we are noticing thoughts and then "letting them go" as we return to our breath (or other anchor)
Statements about the specific practice. For example, after guided visualizations you might write about what you visualized, or after a lovingkindness or self-compassion meditation you might write the phrases you said to yourself, or something else that is kind you can say to yourself now (this type of journalling has a number of benefits for physical and emotional health)
Any challenges and/or benefits that you've noticed about your practice - this is especially good if you write weekly and/or monthly reflections.
Truly there is not right or wrong way to keep a meditation journal. Whatever feels good for you and helps you with your practice is really what matters.
Namaste.