Just remember that being self_employed means that you must also pay the self_employment tax, so include this in your calculations. The self_employment tax is double what you would pay in Medicare and Social Security taxes if you were employed.
Finally, do it again. After you have completed your first month of budgeting, it will be easier to plan for the next month. At the end of the month look at how you spent and make adjustments for categories in which you spent more than you planned, and cut back on the categories that had additional funds in them.