Welcome to LGM, extend.
1 - Give out recordings and words/music ahead of time. Or email a link to a performance online at Youtube, etc.
2- Go over each part by itself, singing/playing a line, then having them sing it back to you.
3- Have sopranos sing their melody on a line, then add altos, then tenors. Also can go the other way - have tenors sing a phrase, then add altos, then sopranos.
4- If you have some strong singers, have them go with their section to another room for 5-10 minutes to go over their part. Then return to the choir to sing it together...
It's good if you can rehearse a song over several practices....