3 Pieces of 30"x20" foam board (I found mine at Michaels on sale for .99 cents. Normally it's only $1.99)
1 White poster board
1 Roll masking tape (The whiter the better. I didn't even come close to using a whole roll)
Lamp of some kind (I had a clip on lamp)
To begin measure 15" on the long side of the board and mark, this should be halfway on the board. Using a box cutter cut the board in half lengthwise. I used the end of another board to help me cut, but any straight edge can be used to guide the knife. I also found it is much easier to cut the board if I measured and cut on one side, flipped it, and did the same on the other side. This piece should measure 15"x20". Make three pieces this size. These pieces will form the top, bottom, and back of the box. Tape edges together. I started just by putting one piece of tape on each side. This makes it easy to adjust the sides, if needed, later.
Next come the sides of the box. Measure two pieces that are 15"x20". Attach them to either end of the box. Again I used just enough take to make it stay on.
After the pieces are put together go around and tape all of the outside edges. Some of the websites I saw recommended that a poster board be placed in the back corner to hide the seam. I did this, it can be viewed in the top picture. I clipped the lamp on the side and aimed the light to hit the top of the box. Below are two pictures I took of items in the light box.
My camera isn't the greatest and as I mentioned I have zero skills when it comes to photography. I still need to figure out the lighting a little bit, but these pictures are good enough for you to get the idea of what a light box can do.