When you are in edit mode in the text of the biography, after you enter a fact like "Mary was born in 1802, she was the daughter of Joe van Gogh and his wife Alice," then click on the little "C" button above the edit box. That will cause
<ref>Insert reference here</ref>
to drop in. Replace the "insert reference here" with the actual reference, (this example is one copied from FamilySearch)
United States Census, 1920," database with images, FamilySearch (https://familysearch.org/ark:/61903/1:1:MNGS-N95 : accessed 22 December 2019), Florence M Evans in household of W L Evans, Washington, Washington, District of Columbia, United States; citing ED 332, sheet 3A, line 19, family 43, NARA microfilm publication T625 (Washington D.C.: National Archives and Records Administration, 1992), roll 213; FHL microfilm 1,820,213.
and then continue with the biography. When you come to the next fact, do the same thing with that reference, and so on. Magically, when you save it all you will have a nice footnoted biography. There are ways to re-use sources without repeating the whole thing everytime, for example if you are using a census to identify a birthdate and place, a marriage and spouse, etc.