It’s a good idea to have your vocalist do proper warm ups before the session. Good warm ups will significantly improve their performance. The thing with vocal warmups, is they involve making a bunch of strange noises, and your vocalist might be nervous, or self conscious to do these warm ups while you’re around. I suggest you play this video for them to follow along with Alexandra on warm ups. Also, have them sing into the microphone and hear themselves through the headphones for the warm ups. This will help them sing more freely, and also get used to the recording setup.