The Major 6th Interval
The Major 6th interval is a really sweet sounding interval that is not so common for melody but harmonically is very pleasing (it's an inversion of a 3rd, but that's for later!).
The easiest way to play a Major 6th interval is to play the root on String 5 with Finger 2 and the 6th will be found 2 strings down and back 1 fret. See below.
There are a few ways to play it and anytime you cross onto or over String 2 (B string) you need to jump the top note up a fret, it's one of the quirks about the guitar tuning. Note that you can also 'reach' for it but that is less common if you're playing what's in your musical mind because jumping out of position can make it harder to continue the idea (until you're an ear ninja!).
There are quite a few possible song references you can use for the Major 6th, we're looking for the first two notes of the melody to use the interval jump of a Major 2nd (Tone).
I usually recommend My Way because it's very well known but Hush Little Baby is equally as good.
Dr Justin prescribes 5 minutes a day of singing a Major 6th (on top of your 5 mins of Play What you Hear). So 10 minutes a day of Ear Training if you can afford it!
1. Start with playing the interval on the guitar and singing along to get used to the sound and feel of the interval. Imagine your Song Reference while you are doing so, and sing the words if it helps. If you're still struggling with singing you can always hum!
2. Play and sing a root note, then stop playing and sing the interval jump and then check it on the guitar so you're sure you are getting it right and in tune.
I would recommend at least a week singing the one interval and using shapes all over the neck so that you get familiar with it everywhere - if there are parts of the neck you find a lot harder - work more on them. Play and sing this until you feel that you can sing it from anywhere and play the interval from any note! Then move onto the next interval!