Build the vocal booth close to the control room of the studio. Vocals are a very emotional way of expression, so it can be helpful for the vocal booth to be close enough to the producer for the vocalist to be in the line of sight. This way, you can communicate what is needed for certain vocal phrases and eye contact will help the vocalist feel at ease during his performance. If he is not comfortable, he can turn around and face a different direction.
Include at least one large window to give the vocal booth a more open feel. Since the booth is very small, it can be very claustrophobic in there and the inclusion of a large window facing the control room will alleviate the closet feeling of a vocal booth.
Carpet and sound-proof your vocal booth thoroughly. In a professional studio it is always best to be able to control the effects that are added to the vocals. Any natural reverb created by poor room construction cannot be altered, so the addition of sound-proofing materials and bass traps is essential. By creating a dead space, you can add the appropriate effects during your mix-down, including lush reverbs and delays. Each project will have different requirements, so the more control you have over the sound of the room, the better off you will be.
Place a high-quality microphone stand in the room. You may need one that is specifically designed for large condenser or tube microphones. Place the microphone near a wall, but far enough away so that the dead-space behind the microphone does not interfere with the vocalist's sound.
Use a Neumann U87 or a Rode K2 for your vocal microphone. These are the highest-quality microphones available and offer the best, truest sound reproduction available for vocals. Combined with a high-quality tube preamp such as a Universal Audio 610 or a PreSonus ADL 600, you will have thick, full vocal tracks that compete with major label-produced tracks heard every day on the radio and on television.