Harmony singing is one of the great pleasures of folk music. It is also, for many singers, the thing that causes the most anxiety. The good news is that the blend you hear on a good record — two or three voices locking together so tightly that you cannot quite tell where one ends and another begins — is not a mystery. It is a set of learnable habits, and most of them are about listening rather than singing.

Start with a drone, not a song

Before you try to harmonise anything, spend a few minutes tuning your ear rather than your voice. Find a drone — a shrutibox, a tanpura app, an open string on a guitar, or a friend who can hold a steady note without wobbling.

Hum gently against it. Then sing a note a fifth above, and hold it. When you are in tune, the two notes stop sounding like two notes and start to ring. When you drift, you will hear a slow throb or flutter — that is the sound of two frequencies fighting each other. Your goal is to make that flutter disappear. Try the same with the third above, then the octave, then come back down.

This is dull work for about a week and quietly addictive after that. Once your ear can find a fifth without thinking, harmony singing stops being guesswork.

Blend comes from vowels, not volume

Most tuning problems in folk harmony are actually vowel problems. Two singers can be singing the same note and still sound sour because one is singing "aaah" and the other is singing "awww".

Take a phrase from a song you know — something simple like the opening line of a ballad — and sing it together on a single vowel. No words. Just "ah", then "oo", then "eh". You will immediately hear where the blend goes thin.

Two useful habits follow from this:

• Shape your mouth the same way as your partner. If they are singing a narrow "oo", do not open out to a wide "aw". Watch their lips if you can.
• Narrow as you go higher. High notes in folk harmony go sharp and shouty when the mouth is too open. Slightly rounded lips and a taller feeling in the mouth keep the pitch stable.

Work out where your line sits

In folk, harmony usually falls into two camps: a line above the melody, often a third or a sixth up, and a line below, usually sitting a third or a fifth under the tune. Above harmonies tend to be brighter and more exposed; below harmonies feel warmer and easier to hold steady, which is why so many singers start there.

The reliable method is to learn the melody first, properly, words and all. Then sing the tune and stop on each long note, hunting downwards for the note that makes the chord underneath feel settled. That is usually your harmony note. Write nothing down at first — let your ear do the work. Once you have found the shape of a line, you will start to hear it in other songs.

Supporting a lead without smothering it

A harmony singer's job is not to be heard. It is to make the lead sound better than they do alone. That means a few deliberate restraints:

• Sing at about eighty per cent of your natural volume. If you can hear yourself clearly above the lead, you are too loud.
• Land on the note, do not slide into it. Scooping up to pitch muddies the tuning of the whole chord, especially on the first note of a phrase.
• Match their consonants and their breathing. Breathe where they breathe, even if you do not need to. Ending a word together is what makes a harmony sound like one instrument.
• Keep vibrato modest. A wide vibrato in a folk harmony can sound like an opera singer in a pub session. Straight, steady notes are the folk default, and a touch of vibrato only at the end of a long phrase is plenty.
• Drop out sometimes. Singing harmony on every verse is tiring for the listener. Coming in on the second verse or the last chorus makes the harmony feel like an event.

Practice routines that actually build the skill

Fifteen focused minutes, four or five days a week, will do more than an hour once a month.

• Five minutes on the drone: hold the fifth, the third and the octave, then move between them slowly without sliding.
• Unison, then apart: sing a scale with a partner in unison, then repeat it in thirds. It trains your ear to hear the interval rather than the individual note.
• Record yourself against the lead. Sing the melody into your phone, then sing the harmony over it. This is brutal and extremely effective.
• Swap parts. If you always sing low, learn the top line. Singers who can move between parts tune far better than those who cannot.
• Rehearse quietly. Singing softly forces you to listen. Loud rehearsals hide tuning faults behind enthusiasm.

Singing in a session

In a session or a singaround, position matters more than you might think. Stand or sit where you can see the lead singer's face and hear their voice, not just the guitar. An instrument fills the ear with pitch information that can pull you off the vocal line, so keep one ear on the singer.

Watch the leader for a nod or a raised eyebrow before you join in. In folk, harmony is offered, not imposed, and the best harmony singers are the ones who know when to leave a verse bare. Get that right and you will be asked back — which, in the end, is the whole point.