I need to budget for one time expense. Can you suggest how

Set aside a small amount each year in a savings account to cover your one time expense.
If this one time expense is for Christmas, then you can do one thing. You can check your past year record and find out how much you have spent last year. You can decide your budget accordingly.

If you have spent $250 last year, then divide this amount by 12. This means you have to save $20.83 every month.

Make an estimate of your one time expense and divide it by 12. Start saving that amount for it each month. Besides, try to trim your expenses in other categories so that you can save more dollars for your one time expense.

Include the expense while budgeting and dedicate a certain amount for that expenses too. You will save a decent amount at the end of the year.