Sagittarius ("archer"), as a noun, is exclusively masculine, but I am trying to refer to a female archer in Latin. Would simply changing the ending to sagittaria suffice?
In this particular case, yes. Sagittārius is the masculine form of an adjective meaning "pertaining to arrows"; the feminine form would be sagittāria.
In general…it depends. Many masculine Latin words are in the second declension, and these can be made feminine by changing the ending to -a. Many others can be treated as "common gender", used as masculine or feminine with no change (e.g. a poeta can be a poet of either sex).
As a rule of thumb, if it ends in -us, change it to -a, otherwise don't change anything. This isn't perfect and will sometimes fail, but 90% of the time it'll get you the right answer.