Harmonising a scale in thirds means pairing every note with the note two letters above it: C with E, D with F, E with G, and so on. Every pair is either a major third or a minor third. Here they all are, from C on fret 3 up to C on fret 10.
The pattern in any major key is always the same: major, minor, minor, major, major, minor, minor, then major again at the octave.
Play the root, then go up to the next string for the third. The shape tells you which kind of third it is. On a single string you just count frets instead: 4 frets up for a major third, 3 for a minor third, as in E–G on the D string.
Next string up, 1 fret back. Root with your 2nd finger, third with your 1st finger.
Next string up, 2 frets back. Root with your 3rd finger, third with your 1st finger.
Tap any note to hear it on its own. All eight pairs, climbing from C on the A string (fret 3) to C on the D string (fret 10). C–E and D–F start on the A string. E–G runs along the D string, and every pair after that starts on the D string with its third on the G string. Lines join each root to its third. Tap a pair below to pick it out, or play the whole scale.
Tap a note on any card to hear it on its own, or press Play to hear the pair. The "Play pairs" switch above sets whether the two notes sound together or one after the other.