Watch the Harmony online guitar lesson by Mimi Fox from Flying Solo
As long as the original melody can be heard, the world of harmony is really almost limitless! Use your ears and see where they will take you. It's very important to remember when using substitution chords that one chord has to lead smoothly to the next chord... if you choose a voicing that is really strange, you have to be sure it resolves well to the next chord. This next chord could be more consonant ("inside") or equally strange! The trick is to be sure that when the entire arrangement is played that it makes sense and keeps the basic integrity of the melody intact. Most successful arrangements have a mixture of consonance and disonance. Too much consonance is boring. Too much disonance is jarring. Explore!