Aeolian mode is easy, because it conforms to the standard minor key signature for any tonic.
Example:
For A, no key signature.
For C, three flats (B-flat, E-flat, A-flat).
For F, four flats (the above and D-flat).
For E, one sharp (F-sharp).
It works for any tonic.
In other modes, it's not always so simple. You have to figure out which chromatics (black-keys) are involved in the mode on a given tonic, and figure it in from there. Sometimes there's no way to do it cleanly with a key signature, and you just have to add accidentals as needed.
Hope that helps.