This is probably the most requested topic that I receive, normally after someone gets a large unexpected expense, or they start thinking about retirement and realize that they have saved a woefully inadequate amount of money.
I recommend using a monthly time-frame to look at your cash inflows and outflows, because most bills are monthly and four weeks is a short planning period that most people can manage. The first thing to do is determine your monthly after-tax income. Usually, this is the amount of money from your paycheck that gets deposited into your checking account. If your income is variable, then use an average of the last three months. (Any savings account interest income would be a bonus.) Next, list out your fixed monthly expenses, such as rent, mortgage, car payment, phone, electric bill, etc. All of these numbers can be changed in the long-term, but first you need to determine a baseline budget of where you are right now.
Make sure you include all of your utilities; some are only paid quarterly or annually, like car insurance, the water bill, or an association fee. Take these expenses and calculate what they would be on a monthly basis. For example,...