How should I journal to lose weight?

Detail. In my practice I encourage detailing - what, how much, when, with whom, what mood - recognizing the fact that we all have situations where we "drop our guards". If you see a pattern of poor food choices in connection with a specific type of situation, you can often anticipate that situation and avoid it, or prepare for it by healthily snacking beforehand to suppress your appetite.
Write down daily. If keeping a food diary, you should be in consultation with a dietician. They can help you decide if you are eating the right proportions of food. Basics: write down what you ate after each meal. Tabulate the calories, lots of apps for that. Review this at the end of the week. Know your BMR and than take 500 calories/day less for each one pound per week you want to lose.