Harmonize guitar parts in songs in a minor key by using a minor third or perfect fifth to harmonize. A minor third harmony is simply three half-steps above the note you are playing in the first guitar lick. For example, if the note you want to create a harmony for is an A, the minor third will be a C note. A perfect fifth is seven half-steps above the note you want to harmonize. Using an A note as an example, the perfect fifth would be an E note.
Build harmony parts around songs in a major key by using perfect fourth or major third harmonies. A perfect fourth is five half-steps from the note you want to harmonize. If your first lick starts with an A note, the perfect fourth is a D note. A major third is two whole steps above the note you are tying to harmonize. Using an A note as an example, the harmony note would be a C sharp.
Use unison and octave harmonies for subtle embellishments of complicated guitar parts that do not require distinctive harmonies. A unison harmony is simply the exact same notes, and in some cases it is referred to as doubling. Octave harmonies are 12 half-steps away from the original note, though they are the same note. As an example, fretting the sixth string at the first fret is an F note if you are in standard tuning. Playing this same string at the 13th fret, which is also an F, would be considered an octave harmony.
Don't limit yourself to any particular rules when working out harmonies. Generally, if it sounds the way you want, it's correct. While minor third harmonies often sound good with songs in minor keys, they can sound quite good in other places as well, depending on the mood of the song or the chord inversion you are playing.