Set up a humidifier in the room before going to sleep to improve vocal quality.
Sleep eight hours. Being tired can weaken the muscles necessary for quality vocals.
Record with a condenser microphone and preamplifier combo set in an area with minimal background noise.
Bring the recording into an audio editing program to remove background noise. For low frequencies below 95Hz add a slight dip of -2dB at 200Hz. To improve the quality of high frequencies raise vocals +2dB at 1200Hz and +1dB around 4800Hz.