The Perfect 5th Interval
The Perfect 5th Interval is for most people the easiest to recognise and is also commonly used so a great starting point.
To play a Perfect 5th, play a note on one of the thickest 3 strings, and then play a note on the next thinnest string, up 2 frets (toward the bridge). See the diagram below for an example.
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.
There are quite a few possible song references you can use for the Perfect 5th, we're looking for the first two notes of the melody to use the interval jump of a Perfect 5th.
I usually recommend Twinkle Twinkle Little Star or the Star Wars theme, but most important is that you find a song that you relate to and that you know very clearly in our musical mind.
Dr Justin prescribes 5 minutes a day of singing a Perfect 5th (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!