I need some help on how to correctely intonate a guitar. I understand that the guitar is sort-of two instruments put together, one is from open to the 12th fret & the other is from the 12th fret to the 24th fret, and that for the guitar to sound right they have to be "in tune" with each other. I was taught to tune the guitar normally and then to tune it from the 12th fret using the screws on the bridge saddles. When I do this though my tuning goes flat or sharp. Is that supposed to happen? Could someone explain to me if I'm doing it right or if there's a different way I'm supposed to do it.