Through a common chord · Modulation · Music theory
This is the classical and smoothest way. You use a chord belonging to both keys, acting as a pivot: the ear understands it in the first, and on seeing what…
Through a common chord
This is the classical and smoothest way. You use a chord belonging to both keys, acting as a pivot: the ear understands it in the first, and on seeing what comes next reinterprets it in the second. From there, a cadence in the new key confirms the change.
Closely related keys have several common chords. C major and G major share C, Em, G and Am, but the most effective is the one with a subdominant function in the new key, because it leads naturally to its dominant: Am is VI in C and II in G.