In my opinion, if you can fully understand how to use thirds on your guitar, you've come a long way... Thirds are one of the best sounding harmonies to the human ear, and there are many, many different ways in which they can be applied on the guitar.
In these guitar lessons I try to bring out elements of that, but to really understand it you're going to have to dig into this on your own too. I've posted videos on thirds in the G major scale, and inverted thirds in G, and today's guitar lesson looks at some thirds off the second and third strings in D major.
If you can't see my hand very well, the only two patterns I'm using are the minor third (0-0-0-4-3-0) and the major third (0-0-0-5-5-0). Take those two patterns, and move them up the fretboard, and you'll find what I'm working with.
If that's all completely Greek to you, I highly recommend checking out my course on Guitar Scale Patterns, which will really open up the fretboard for you.
The big takeaway from this lesson is recognizing the two different thirds patterns, major and minor, and then recognizing how those relate to the various scale patterns up and down the fretboard. Once you tie them all together in your mind, the whole fretboard is your oyster...
Thirds in D
Watch on Youtube