Watch the Guide Me online guitar lesson by Brad Carlton from Chord Studies: Open String Chords Vol. 1
Guide tones refer to thirds and sevenths. Major chords use a major third and major 7th while minor chords usually use the minor third and a minor 7th. Study these shapes and work out where the thirds and sevenths are. Take note how you can move through different families of chords simply by altering the thirds and/or sevenths.