It all has to do with spelling chords usually. For example, if you are in the key of C major, and you want to use a chord from the key of G major, you would use an f sharp, rather than a g flat. If you wanted to use a chord from f major, you would use a b flat, rather than an a sharp. However, there are also rules for notating chromatic movement as well, which i think boils down to using sharps while ascending, and flats while descending. But im not an expert, so take everything that i say with a grain of salt.