There are two methods, the default "new style" which gives you a list of all your categories and allow you to enter a budget amount for each which you copy to future months. And the old style "mixed interval" style which gives you a form to create a list of your budgets where you set a start date and end dates as needed.
It's all documented here:
Please note that you won't be able to budget a loan payment if you have the loan set up in MD. But there are methods to work around that issue.