Twinkle Twinkle works well for this. On a single string (high E), try: 0-0-7-7-9-9-7 / 5-5-4-4-2-2-0. Slow, one note at a time, sing along while you play. Once that feels easy, do the same melody starting from a different string. Same shape, different starting note. Your ear will start hearing the intervals, not just the notes. Mary Had a Little Lamb and Ode to Joy work the same way and use mostly the same notes. Three songs, one technique.