Measure the area where the vocal booth is going to be located. The smaller the area, the better. Lay two layers of SheetBlok under the floor plate.
Place 2x6 floor joists into U-boats perpendicular to the floor, 16" to 32" centers. Between the joists place 4-inch mineral fiber insulation. Floor joists can also be 2x2 or 2x4 depending on the ceiling height.
Build the floor layers, also known as a floor sandwich. Construct the first layer of 3/4" plywood, followed by Sheet Blok, 3/4 MDF Board, then floor finishing. Whenever possible, try to glue the various layers instead of nailing. The reason for this is that the adhesive itself will act as sound isolation. Nails and screws act as bridges and tend to transmit sound between layers. If you have to use either of the two, use screws because they cause less resonances.
Build the wall layers. If building within an existing room, assure that the space from the existing wall is at least one inch, with two inches being preferable. Erect 2x4 studs 16" on center, stuffing 2" of mineral fiber insulation between. Then layer 5/8" gypsum board followed by SheetBlok and 1/2" gypsum board. Place a StopGap between the drywall and floor.
Attach the door. The best to use are solid-core, slab, wood doors. These should be flat and free of moldings. For extra isolation, you can add a layer of SheetBlok to either side before attaching the doorknob. Glue rubber door seals below the door and around the edges to control sound leakage.
Treat the walls and ceilings with acoustical absorption foam (which controls flutter echo and slap) and diffusers (which keep sound waves from grouping and widens the "sweet spot").