Make a yearly list
Look back over twelve months of statements and calendars. Include insurance premiums, registration, memberships, gifts, maintenance, travel, and medical costs that did not happen every month.
We cover this in more detail in our guide to How to Make a Monthly Budget From Your Bank Statements.
Create small monthly buckets
Estimate the annual amount and divide it by twelve. Keep the money in a separate savings space or track it clearly so a high balance is not mistaken for money available to spend.
Expect estimates to move
The goal is not perfect forecasting. Add a little margin, review after the expense arrives, and update the monthly amount for the next cycle.
For a deeper look at this topic, see our full guide to How to Create a Sinking Fund for Irregular Expenses.