A third above means your note is two steps of the scale higher than the tune's note, and stays two steps higher as the tune moves. In C major, when the tune sings C you sing E; when it sings D you sing F. The gap is sometimes a major third and sometimes a minor one, and you do not have to think about which: if you stay on the scale, the scale decides.

It is the first harmony everyone learns because it is the closest one that does not clash. Close enough to feel like part of the tune, far enough to sound like a second voice.

Hear it

The lower line is the scale; the upper line is the same scale started two notes higher. Solo the upper line, sing it a few times, then bring the scale back in and keep singing.

Finding the first note

Do not find it from the tune. Find it from the key. If the tune starts on the tonic, the note a third above is the third of the scale, which is the note that tells you whether the key is major or minor. Sing the tonic, sing up two steps, and hold that. Then start the tune and keep holding it until the tune moves.

Once you have the first note, the rest of the line is the tune's shape, shifted. Every step the tune takes, take the same step.

Where it breaks

On leaps. When the tune jumps a fifth, your line jumps a fifth too, and a fifth is harder to land than a step. When it breaks, it is nearly always on the leap, and nearly always because you landed on the tune's note instead of yours. Practise stepwise tunes first, then tunes with small leaps, then wide ones. That is the order Blend's lessons use, and each one grades every note so you know which leap got you.