I was a former member of my college choir, and some of the techniques I learned during our rehearsals are breathing exercises, humming, and pitch matching.
For breathing exercises, you inhale through your nose for 4 seconds then exhale for through your mouth for 8 seconds. Do this for about 2 minutes.
For humming, first hum notes with your mouth closed then eventually open your mouth. Do this for about 2 minutes.
For pitch matching, we always have a piano. The conductor plays a chord then you match it with your voice. We do this for like 5 minutes for each person. If you're alon, listen to piano chords and sequences and copy them! Hope this helps
