Almost everyone who starts singing harmony hits the same problem. You know your part on its own, then the tune comes in, and a few notes later you're singing the tune. That's normal: the tune is louder in your head because you've heard it all your life. Holding your part is a skill, and you build it in steps.
Learn it alone first
You can't hold a part you only half know. Before anything else plays, learn your part on its own until you could hum it from memory.
- Listen to it a few times, following the words.
- Sing along quietly with it.
- Sing it without help, and check yourself against it.
If this step feels slow, it's working. Most trouble with holding a part comes from skipping it.
Then add the tune, quietly
Now sing your part with the tune playing softly underneath. The tune will tug at you, but at a low volume you can still hear yourself clearly. Stay here until you can get through without slipping.
Then turn the tune up
Bring the tune and the other parts up, step by step, until they're as loud as your own part. Each time you turn them up, you'll feel the pull again for a little while, then it fades. When you can hold your part with everything at full volume, you've got it.
Find your anchor notes
Every part has a few notes that are easy to find and hard to lose: a long held note, the first note of a phrase, a note that's the same as the tune's. These are your anchors.
Learn where they are. If you wobble in between, you know a safe place is coming, and you can aim for it. Many singers mark them in their head: "I start on the same note as the tune here", "this long note is the one under the word 'night'".
Listen across, not just inward
It's tempting to block your ears and sing harder. That usually makes things worse. Instead, listen to how your part fits the others.
- Notice when you sing the same note as another part, and when you're a little way apart.
- Notice when you move together, and when you hold while the tune moves.
- Listen for the blend. When your note fits, the sound settles. When it doesn't, you'll hear a slight roughness.
The more you listen across, the more the other parts start to help you rather than pull you.
When you drift
You will drift sometimes. Everyone does. What matters is coming back.
- Stop, don't push on. Singing the wrong note louder only teaches it.
- Find the next anchor. Come back in on a note you're sure of, at the start of a phrase.
- Go back a step. If one phrase keeps catching you, practise it alone again, then with the tune quiet, then louder.
- Slow it down. A tricky phrase is often easy at a slower speed. Get it right slowly, then bring the speed back up.
Try it in Stackapella
Silent Night is slow and gentle, and the alto stays a third below the tune most of the way, so it's a kind first song for holding a part.
- Listen to your part on its own, a phrase at a time, with its guide voice.
- Sing along with the guide while the other parts come in. Turn the others quieter, or mute them, and bring them back up as you get more sure.
- Go On your own: your guide switches off and the others keep going. Watch your notes light up as you hit them, so you know straight away when you've drifted.
- Loop any phrase that catches you, and slow it to 85% or 70% until it's easy.
- When you can hold it, Record your part, at full speed, with the others.