Use a tape measure to determine where to cut your acoustic foam sheets in order to make four sheets that will fit within four sides of your 12-by-12-inch container.
Mark each 12-inch measurement with a permanent marker. Once the measurements are complete, draw a straight line to each mark.
Cut the acoustic foam padding along the straight lines with a serrated knife. When complete, you should have four equal-sized foam blocks 12 inches square.
Stand the container upright and place each of the four pieces inside. Place one against the back of the container, two on each side, and slide one into the top of the container to act as a roof. Trim down the pieces as needed to fit snugly.
Choose a microphone to capture your vocals and place it inside the bottom of the container facing the opening. Cardioids are effective for capturing vocals as they are built to pick up sound directly in front of the microphone. If you are recording directly into a computer, a microphone with a USB input will make hookup very simple.
Choose an appropriate recording console. If you're a musician and play with a band, multitrack recorders are expensive but allow for simultaneous recording from multiple microphone inputs and an array of onboard effects. PCs and Macs are also workable recording consoles for singer-songwriters, bands or a speaker hosting a podcast, with hundreds of recording software titles to choose from.
Set up a sturdy table to support the microphone booth and a comfortable chair that supports good posture If you choose to sit while recording vocals. If you choose to stand, the microphone booth can be set on a shelf or any sturdy perch that puts it at your mouth level.
Choose a pair of headphones to enable monitoring of the playback from your recording console or computer, which you will connect to the "headphones" or "output" jack. Headphones vary wildly in capability and therefore price. Consider over-ear headphones as opposed to earbuds as they are better insulators from outside noise and will allow you to hear the subtleties of your vocal takes more clearly.