Choose a simple format
Use a notebook or a document you can open easily. Begin with the date, a short description and the amount for items you want to understand. Avoid recording account passwords or full payment-card details.
Use categories you recognize
Choose a small set of labels that make sense for your household. If a purchase fits two categories, pick one consistently rather than creating an elaborate system. The point is to make your own records easier to read.
Look for questions
After collecting some entries, note what you want to understand better. A record can reveal questions about repeated purchases or timing, but it does not by itself tell you which expenses to change.