Choose a small enclosure to function as your booth. A room or closet large enough to fit one or two people comfortably will work. Using a small room is optimal because it prevents damp, distant reverberations off surrounding walls. If you are constructing the room yourself, a six-foot-by-six-foot room would be most desirable. Some 2x4s and drywall or even some hardwood paneling will do just fine for the walls. The room doesn't have to be hurricane-proof to meet your purposes.
Take notice of the sound of your vocal booth, making sure it suits your needs. Sometimes bathrooms and other small utility rooms can create a pleasant acoustic effect on the human voice, and it may be something you wish to keep. Some rooms may do the opposite. Also keep in mind that parallel walls can cause sound phasing issues, making it difficult to determine how your vocals will actually sound when recorded. If this is an issue, hang a few bits of soundproofing foam; if this is not available, covering the walls with curtains or even just hanging a few pictures can help deaden the sound of a room and kill unwanted noise. Carpet the room if it isn't already carpeted to prevent sound from bouncing loudly off of concrete, wooden or tile floors.
Create or find an opening in the room for non-verbal communication between the vocalist and the sound engineer. If possible, the opening should be in the engineer's line-of-sight from the mixing console. A small window at head-level to the vocalist (roughly four to seven feet up) is suitable. In a home studio, most people tend to work with what they have at their disposal, and simply leaving the door to your "vocal booth" open may be the only option.
Choose a spot in the vocal booth through which to run your microphone and headphone cables. Choose a spot near the floor, preferably under the window if your vocal booth will be a permanent fixture. Even in a makeshift booth, this is important to prevent injuries and broken cables that can result from tripping. Duct taping a portion of the cable to an out-of-the-way spot on the floor or wall, leaving enough slack to remain useful, can prevent such accidents. Be sure you also leave some extra room to run a lamp through so you don't have to spend extra time and money wiring the room for electricity. Drill holes that are just big enough for your cables.