I built one in my studio.
1) get the foam from here
http://www.silentsource.com/afoams.html , the prices are good, the shipping is fast, and they really help you get exactly what youre looking for. I got 450 square feet of foam for $300, and they accidentally sent me 600 square feet.
2) Applying the foam to the walls is a beeyotch. We first went with Elmer's spray adhesive, which says on the back that it works for foam. The foam stayed on the walls for about two weeks and fell off one piece at a time over a week period til it was all off. Then we got Foamtak, which is made especially to hold up Acoustic Foam (is available at most major music stores, check Guitar Center or whatever). This stuff was the opposite. It was like rubber cement. the foam stayed up like no one's business, and now that we are moving out and upgrading to another studio across town, it is ridiculoulsy difficult to remove. The foam tears when you take it off, and no matter how gentle you are, or what method you use, leaves foam residue and glue on the walls. I dont recommend Foamtak unless you have a fairly permanent spot for your booth. At the new studio, we bought plywood sheets and measured them to fit the walls of the booth. We then applied the foam to the sheets, perfect fit, and just nail the foam-covered plywood to the walls of the booth. Works perfectly, and removing the foam will be a matter of removing a few nails from each wall and lifting the plywood up and out.
3) Go to your local music store and get a wall panel that goes through the wall of the mike booth, leading out to your control room. this allows you to put plugs for headphones and mikes, without having to run them through walls or ceilings, or worse yet, under the crack in the door.
4) soundproof the crack under the door. Easiest way is to buy a door hinge, screw it to a 2x4 covered in foam, and latch it closed when the booth is occupied, covering the crack. We didnt have our previous doorcrack soundproofed, and a few otherwise perfect takes picked up residual noise from the control room on the mike. Sucked.
5)Get a light for the booth, so MCs can see their notebook. If they have to struggle or squint, or worse yet KEEP TURNING THE PAGES CLOSE TO THE MIKE to find their spot, you will do lots of unnecessary takes. Make sure it is not one of those cheap $20 vertical lamps with a dimmer, as dimmer lights tend to generate a 60 Hz hum that is audible on the mike (since its in the mike room).
6) Make sure MCs leave their cellphone in the control room and dont take them in the booth. Also make sure that the booth is air conditioned or cooled (soundproof=airtight), as it gets pretty hot in there. Make sure whatever cooling system you have does not pick up on the mike; central cooling is cool, swamp coolers and large vents are a nono.
7) If possible, have a set of monitors wired into the booth. What slowed us down was the fact that MCs would constantly take off the headphones, and open up the door to hear the playback, and when they came back, their mike positioning had changed slightly. So we had to constantly reposition mikes and re-check mike levels after each take. Having a set of monitors for playback in the booth allows them to hear the playback without moving (which will save you time and headaches).
That's about all I can think of.
