One could spell out the numbers:
spell out: 2. to write or print in letters and in full
Both AP and Chicago appear to use this terminology, as does MLA. This means more or less the same thing as write out, but if this is insufficiently specific for you, you could simply say to write numbers using words or to write numbers out longhand, with the additional caveat that longhand in research means the full number as opposed to its scientific notation (e.g. 25000000 instead of 2.5 × 107).
When we express a number using figures, we write with numerals.
numeral: a sign or symbol that represents a number.
If you want to be even more specific, you can specify the type of numerals: Egyptian, Mayan, Babylonian, and so on, though the only systems commonly used in primarily English-speaking countries are "Arabic" numerals (actually Indian in origin, but commonly known as Arabic) and Roman numerals.
Alternatively, you can specify that it be written using digits, or numerical digits if there could be confusion about the type of digit in use.