The first step in creating a DIY sound booth is picking a location. The size of a room greatly affects recording. Small rooms have less room for sound to move around in, which will result in a cleaner, less echoed recording. Shy away from rooms with tile or linoleum floors, like kitchens and bathrooms, which will reflect, rather than absorb, sound. Carpeted floors dampens sound better than wood. Your best bet, especially if you're recording only yourself, will be a closet full of clothes.
Next, you'll want to dampen your chosen room. Dampening sound, simply put, disperses the sound energy. For a performance venue, this will keep the sound in or, in the case of a recording studio, will keep other sounds out, protecting your takes from unwanted noise from neighbors or the street. For thrifty DIY sound dampening, use a mattress to cover the door, a big weak spot for soundproofing. Windows are another weak spot, and will allow a significant amount of sound through, especially if they contain only one pane of glass. Cover them with pillows, or thick blankets or drapes. You can also cover yourself and your microphone with thick blankets.
In your improvised sound booth, be it a closet full of winter clothes or a fort made out of pillows in your living room, make sure to set up your microphone for optimal sound. Mount the microphone on a sturdy surface or stand that won't vibrate when you speak. Use a pop filter, a small shield that reduces popping noises in speech. Pop filters are relatively inexpensive (http://www.amazon.com/Nady-MPF-7-Mic-Pop-Filter/dp/B000FA38VI), but can be replicated for almost no cost by wrapping tights or spandex over a coat hanger. Whatever you use, make sure it is not in direct contact with the microphone.
Finally, get comfortable. It's crucial that no matter how you set up your sound booth, you're willing and able to easily sit or stand in the booth, and keep a consistent distance from the microphone.