Watch the Chord Substitution: 3 online guitar lesson by Fareed Haque from Bebop Improv Survival Guide
Principle 3: Any Dominant can become a ii-V.
Getting hairy now: The key here is that a ii-V is simply a glorified, jazzified classical suspension. Dm7 has color tones of F and C. G7 has color tones of F and B. So really the only thing that changes between the two chords is the root of course and the C in Dm7 moving to B in G7. C to B against G7 is a classic suspension and resolution…SO our ii-V-I, Dm7-G7-Cmaj7 can become D7-G7-C7 using Principle 1.
Then we can turn each of these dominants into a ii-V using our principle 3: D7-G7-C7 becomes Am7-D7 to Dm7-G7 to Gm7-C7.