Watch the Harmony Lead-Slide: 4 online guitar lesson by Robbie Calvo from SoloCraft
All of the slide techniques we have learned so far still apply to this example so remember the muting techniques, etc., while learning to play this melody. This is a simple idea but don't underestimate the power of phrasing. You should also be very aware that if you intend to harmonize a melody you will have to match the phrasing of the melody when you play the harmony part. So, make sure you like your phrasing on the melody part before trying the harmony part.
Take a look at the 6ths that are the harmony to our melody. The shapes have been laid out for you in a linear fashion on fret board diagrams. Watch the video and learn what I am doing with my fingers. I am playing the whole part using double-stop sixths first so you can see the position of the notes before translating that to a single note slide part. Let's play the harmony part now a 6th above the melody using the slide.