Cut your boards to size. It's essential that your booth walls are of unequal width. This is because sound reflects more easily from parallel surfaces. Having walls that are slightly longer and angled away from each other is a good method of eliminating sound reflection. Saw an inch form the width of the first board. Two inches from the second board and three inches from the third board.
Dress the interior of the boards. Staple or nail on a sheet. You can use an old bed sheet if you wish. Cover the sheet in glue and then stick on an even layer of wadding so that wall is cushioned. Cotton wool, torn-up old clothes and rags are fine.
Staple on a second sheet over the wadding. Glue on your foam pieces at irregular intervals around the board. If you'd prefer not to use foam, use the bottom half of an egg box. Ensure that the area of the board around head height is densely covered, but aim for an irregular distribution. These foam pieces will deaden the sound inside the vocal booth and will make recording the voice an easier task.
Attach the locking hinged legs to the back of each board so that they can be freestanding. Screw in the bracket before attaching the hinge. Fix the leg at a different height on each board so that each one stands at a slightly different angle. The metal leg should swing out and support the wall as it leans back. The leg should be attached so that it swings out no less more than a 20-degree angle.
Position the walls so that they face each other. There will be a gap between each board as you are not fixing them together. Make one gap large enough so that the singer can get in and out of the booth. The cushioned side of the wall should be facing inwards.
Place a rug on the floor if the room is not carpeted and drape a sheet over the top to act as a ceiling. Now drape a sheet over the gap between each board to further insulate the sound.