Jazz musicians use a little trick that seems to also work with rock and pop music - perhaps other styles, too. Assuming you know all the chords, you would concentrate on the ii (two), V (five), and I (one) chords of any particular key. In your example of the key of C, the ii will be a d minor chord and the V will be G major. The typical progression will end with ii-V-I (in this case, dm-G-C). Learn the ii-V-I in every key and then notice that some keys share the exact same chords. These shared chords can be opportunities to substitute chords from other keys.Take this sample progression: C-am-dm-G-em-am-F-G-C. We'll use chords of that progression as a guide for substituting ii-V-I progressions from other keys, which might give us instead: C-am-D-G-cm-F-Bbm-Eb-Ab. am-D-G is ii-V-I in the key of G, cm-F-Bbm is ii-V-i in the key of Bbm. Bbm-Eb-Ab is ii-V-I in the key of Ab. I hope you can make use of this info and that you enjoy exploring this fascinating aspect of music theory!