Plug your audio interface into your computer, usually through a USB port.
Open Reason 4.0. Plug your microphone into your audio interface. In Reason, go to your mixer window. Set the input for the audio interface on the track you want to record your vocals.
Set your master fader output in the mixer view to your headphones. This will let your singer hear the music and his vocals at the same time.
Go to the track view in Reason. Set the vocal track in record-enable. This is usually a button with a circle or the letter "R" on it. You are now ready to record.
Look at your transport, which is a window with symbols like those you would find on a CD player or tape deck. You want the button with the circle on it, which is the record button.
Tell your performer to be ready. Press the record button on the transport. Have the performer sing the vocal parts.
When the performance is over, press the stop button on the transport. This is a button with a square on it.
To listen back to the performance, go back into the mixer view. Change the output of the main master fader to your computer speakers. Now you can listen and decide if another take is necessary.