I don't do it as often any more but I did it for about a year and it really helped.
I wrote down everything I ate: how much, what time it was, who I ate with, where I ate it, and how I was feeling (my body/mind/emotions etc.). I also wrote down all my BM's and any time I had cramps or bloating etc., including any connections I made to those feelings with what I had eaten or done that day. I also wrote down all exercise (I was exercising a lot then), including cardio, weight lifting, yoga, even short walks. Basically any time something was going on with my stomach, it got written down in that journal!
I used FITDAY.COM for this and it made it really easy! I highly recommend it. I still use it just not as much any more.