Lorem ipsum dolor sit amet, consectetur adipiscing elit. Sed do eiusmod tempor incididunt ut labore et dolore magna aliqua, where each voice carries its own melodic line. The move from a Cmaj7 to a G7 is less about the chords than about the four paths between them.
The four voices
Ut enim ad minim veniam, quis nostrud exercitation ullamco laboris nisi ut aliquip ex ea commodo consequat. Each of soprano, alto, tenor, and bass occupies its own register, and the art is keeping all four independent while the harmony underneath them changes. Duis aute irure dolor in reprehenderit in voluptate velit esse cillum.
Common-tone retention
Excepteur sint occaecat cupidatat non proident, sunt in culpa qui officia deserunt mollit anim id est laborum. When two chords share a pitch — say the note shared by Fmaj7 and Am7 — leave that pitch in the voice that already has it and let the others move. Sed ut perspiciatis unde omnis iste natus error sit voluptatem accusantium doloremque laudantium.
Contrary motion
Nemo enim ipsam voluptatem quia voluptas sit aspernatur aut odit aut fugit. When the bass falls, let the soprano rise; opposing directions keep the texture open and sidestep the parallel fifths that flatten four-part writing into two. Neque porro quisquam est qui dolorem ipsum quia dolor sit amet, consectetur.
Resolving the tension
Quis autem vel eum iure reprehenderit qui in ea voluptate velit esse quam nihil molestiae consequatur. The leading tone wants to rise a semitone to the tonic and the chordal seventh wants to fall; honour both and a G7 melts into its resolution without anyone noticing the mechanics. At vero eos et accusamus et iusto odio dignissimos ducimus qui blanditiis.
If the geography of these chords is unfamiliar, start with The Circle of Fifths, Practically and come back. For the historical conventions behind these rules, the Wikipedia entry on voice leading is a reasonable map. Temporibus autem quibusdam et aut officiis debitis aut rerum necessitatibus saepe eveniet ut et voluptates repudiandae sint et molestiae non recusandae.