Mark four out of the five poster boards to create a large middle hole for each one. This hole should start 4 inches from the left, right, top and back contours of the board. For a 20-by-20-inch poster board, each large middle hole will be 16 by 16 inches in size. Leave the fifth poster board as is.
Cut each hole for the four marked pieces of your poster boards.
Fold 1 inch for the two opposite sides and one adjacent side of each of your poster boards.
Attach the first three boards together through their folded parts to make the left, right and back sides of your box. Use a glue gun or staple gun to securely connect them together. As a guide, allow the back side panel's folded parts to attach from outside the left and right panel's folded parts. This means the folded parts of the left and right panels are set inside the work-in-progress box.
Attach the fourth board at the top of the box. Allow its three folds to attach at the exterior of the box. The part without the fold should be the one placed at the box's front.
Attach the fifth and final board at the bottom of the box. This should be the board that doesn't have a middle hole on it because this board will serve as your box's base. Allow its three folds to attach at the exterior of the box. The part without the fold should be the one placed at the box's front.
Use thin, white cloth or bond papers to fully cover each hole of your newly assembled box. This helps diffuse your light sources during the shoot. You can attach these using a staple gun, glue or adhesive tape. You may prefer to have them permanently placed over the holes, but using them as temporary covers makes your box more versatile for varying lighting requirements. For instance, you can replace the white cloth or bond papers with red or blue gels for different lighting effects.
Position the subject you will shoot inside the box, and set up your lights anywhere you deem necessary to achieve the lighting design you want. For instance, you can place your main source of light on the upper left side of the box, then a fill light on the middle right side of the box. You can also add another supporting light source on the lower front of your box. Rearrange each of your light sources' height, angle and distance based on how you want the subject inside the box to look in your photo.