Watch the Fareed's 2nd Principle of Chord Substitution online guitar lesson by Fareed Haque from Jazz Comping Survival Guide
A ii-V can be simply described as a fancy suspension. We create a suspension by replacing the 3rd with the 4th. The guide tones of G7 are B, and F. The guide tones of Dm7 are C and F. Since C is the 4th of G7 a Dm7 could be called simply a Gsus with the D (5th) in the bass. Well maybe not so simply, but check it on the guitar. The key here is that a ii-V can be used almost anywhere - a 7th chord can become a ii-V - since we are simply creating a suspension and resolving it. Guidetones of Dm7 - C, F - resolve to G7 - B, F.