Income first
Use the income that is realistically available during the planning period, especially when pay varies.
Separate obligations from choices
Housing, utilities and required payments behave differently from flexible categories such as entertainment.
Include non-monthly costs
Annual fees, repairs, school costs, gifts and seasonal bills can break a budget if they are ignored.
Compare plan with reality
At the end of the period, compare actual spending with the plan and revise assumptions rather than treating every variance as failure.
Educational information only. Use current product terms and the rules that apply in your country. For decisions with significant financial consequences, consider an appropriately qualified professional.