A document outlining your needs for two years vs. your abilities.
I would include a detailed budget for the time you will spend in school, and a statement of what portion of that would be covered by your savings, stipends, grants etc that you have secured. Then make a pitch on how much money you need to cover the rest.
Make sure you budget things like your health insurance, transport, travel, books, etc. If you have children/spouse - they get special treatment in your statement as they are usually viewed as an extra burden.
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