Plan income minus outgo to zero
Start with your monthly number from Day 1. Assign amounts to each category until nothing is left unassigned. Leftover money gets a job too: savings or debt.
Add sinking funds
Car registration, holidays, annual subscriptions and school costs aren't surprises; they're just not monthly. Divide each yearly cost by 12 and set it aside every month.
Keep a small buffer
Plan a little room in groceries and gas. A budget that's too tight usually gets abandoned in week two.
Do this today
Write next month's plan so income minus planned spending equals zero, including at least two sinking funds.