A good musician can instantly play any interval from any note either harmonically (at the same time) or in ascending or descending order. 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 only 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 each individual string. This is easier than it sounds. 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.