You might want to look at the pictures at the end before you start cutting.

What you need:
A cardboard box. (Mine is 34 cm high and 25 cm wide, which would be a little over a foot high and a little less than a foot wide.)
White paper (printer paper is good, as long as it's not super shiny).
Layered white napkins, or sheer white fabric. (If you're using a napkin, pull the layers apart so that you only have one layer pieces. I used 1 three-layered napkin to make this.)
Tape.
White paint. (Optional.)
Straws or something similar. (Optional.)
Glue. (Optional.)
How to do it:
1. Cut off the top of your cardboard box.
2. Cut a large rectangle out of the front. Leave maybe half an inch at the top and the sides, cut the bottom close to the bottom of the box.
3. Do the same to the sides, but it's okay to leave more at the bottom here.
4. If your box seem a bit wobbly at this point, tape straws or something similar to the front "pillars" and the backs of the top "bits". It's okay to use a lot of tape. (You can see the insides of my top "bits" in picture 1.)
Note: Step 5 and 6 are not as far as I can tell actually necessary, but I do like being thorough.
(5. Dab white paint around the edge of your back and the inside of the bottom. (Unless you're using a white box, of course.)
6. Cover the back and bottom with white paper. I glued mine. I'm sure you can as well tape it. (See picture 1 for a look at the inside of my box without the "extra" paper.))
7. Tape or glue napkins or fabric over the holes in the sides. They should be completely covered. Tape them from the outside. (See picture 2.)
8. Tape or glue a napkin over the top of the box to make a "ceiling." (See picture 3)
9. Depending on the size of your box you might want to put a "peg" at the bottom of the box to keep your paper from slipping.
10. Put a white paper into your box without folding it. (See picture 4.)
11. Position a light (or two) in a way that creates minimal shadows. (I put my desk light on a stack of books to light from the top.)
12. Take pictures.
Picture 1:

Picture 2:

Picture 3:

Picture 4:

And for comparisons sake pictures of a cube take with and without the use of the light tent:
Without:

With:


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All pictures are taken with my cellphone camera and have not been altered in any way. The front picture taken without the light tent was too horrible to even post, since my camera insisted on using the flash. The cube is indeed orangeish, not pinkish.
Thanks for reading and I hope someone finds it helpful. Making the tent took less time than writing this post.



































