The Major 2nd Interval
The Major 2nd is often called a Tone or Whole Step, two fret step on the guitar neck (2 semitones). It's easily recognizable as the first 2 notes of a Major Scale (Doh, Ray) and there are many songs you can use for reference too!
The easiest way to play a Major 2nd is to just go up 2 frets (so there will be one 'empty' fret between the note you start on and the one you end on) and you can also 'step back' onto the next string.
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 Major 2nd, we're looking for the first two notes of the melody to use the interval jump of a Major 2nd (Tone).
I usually recommend Happy Birthday because that seems to be an internationally recognized melody, 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 Major 2nd (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!