Budget math is simple: The amount of money going out must always be less than, or equal to, the amount of money coming in. If you’re like most people, incoming funds are generally predictable and easily documented—your paycheck and earnings on savings and investments.

Money going out, however, may be a bit fuzzier. Sure, you write checks for bills and you know about how much you’ve spent using credit cards, but cash may seem simply to disappear. As a result, you don’t have a good overview of where your money goes. Understanding and changing your spending habits is the key to successful budgeting. But brace yourself. When you sit down and force yourself to make an expense record, the evidence against you is in black and white.

Write down all your living expenses: rent or mortgage payment, utilities, transportation and so on. The monthly expense record that follows is a starting point. Some items on this list may not apply to you, or you may need to add other items.

To make your budget work, you’ll need a record of where all your money goes. Keep a money diary every day or every week for a month. Document every purchase by getting a receipt for everything you buy, from the cup of coffee at the drive-up window on Monday morning to the movie tickets on Saturday night. If you don’t get a receipt, keep a note pad handy to record the expense. Keep track of how you pay for things, too. Note whether you pay cash, use a credit card or write a check.

At the end of the month, fill out an expense record. For items you pay just once or twice a year (like semiannual car insurance payments), simply divide the payment by the number of months it covers and write that amount on the monthly record.