As well as understanding how intervals work, you need to be able to instantly play them on your instrument either ascending, descending, or harmonically from any given note.
The diagrams below show the most common ways of playing intervals on the guitar. Learn them one at a time and listen carefully to the sound of each interval as you play it.
These diagrams show the most common ways of playing intervals on the guitar, but they are not the only patterns. You should also work on playing every possible interval on one string. This is fairly easy because all you have to do is work out how many frets apart the interval is: e.g. a perfect 4th is five frets apart, a minor 6th is eight frets apart, etc.