After playing Portal, I needed my very own companion cube, but didn't want some cube cluttering up my desk. So after spending some time in the hobby store, I found a nice cube shaped tissue box holder, and thus my useful companion cube was born.

What you'll need:
Cube tissue box holder (this is not a plain tissue box since tissue boxes will need to go inside of it)
Silver high gloss spray paint
White high gloss spray paint
2 Cereal Boxes
Scissors
Pink Acrylic paint
White acrylic paint (for touch-ups)
Paint brushes
4x 2 inch wooden disks (can be found in Michael's, Hobby Lobby, etc)
Hot glue gun
Extra hot glue
Razor
Pencil

Step 1: Base Paint and Pattern Preperation

Take your tissue box outside and spray paint it with the gray/chrome spray paint.

While that dries, print out the pattern and cut it out.

Cut along the edge of your cereal boxes so that you can open them up and lay them down flat.

Trace the pattern 8 times on the inside of the cereal box. You should be able to get 4 on each box (2 on front, 2 on back). Add in the score lines.

Go outside and put a second coat on your tissue box.

Step 2: Cutting Out the Corners

Cut out your patterns.

Score the cardboard on the penciled lines using your scissors. This will give you a much nicer corner. Score along all your drawn lines.

Now fold along all of these lines. Fold them so that the plain brown cardboard is on the INSIDE, not the outside like I have shown in the following pictures (this was a mistake I corrected later). You can use tape to do this, however, REMOVE the tape before spray painting.

Now, glue the inside to secure the two outer edges. Remove the tape. It should stay together without the tape.

It's easiest to glue the other two corners first, like I've shown in the last image. Then glue the remaining flaps. Just fold them down towards the inside until they touch, then glue them together.

Repeat this 8 times.

Step 3: Paint and Attach the Corners.

Now, take your 8 corners outside and spray paint them white. This will take at last 3 coats until they are pure white. Do not try to do it in one coat, you will only create drips in the spray paint. You can usually spray another coat after about 20 minutes (depending on your conditions outside).

Also, spray paint your 4 two inch wooden disks white. 3 coats should cover up the wood grain.

Go ahead and give the box another coat of gray.

Allow everything to dry. Unless you want finger smudges all over your perfectly white pieces, let them dry for at least 2 hours, if not overnight.

I attached the 8 corners first, then applied the pink paint, however it's probably best to paint your pink lines first. Both methods will come to the end result.

Now, put hot glue on all the contact points of your corners, then attach them to the box. You will need to do one corner at a time and attach the top ones first. You will need to work fast before the glue cools.

The bottom ones are a little more difficult to attach because of how the box is constructed. In order to be able to insert tissue boxes later, you will need to attach the bottom boxes carefully. I had to put glue only on the areas where it would contact the outer faces, and not on the bottom. Then I had to use a razor blade to cut away the excess corner, so that later, I can put a tissue box inside. Other wise the corners block the bottom entrance.

After cutting away the excess corners, glue down the remaining contact points. I also folded up scrap pieces of cardboard and stuffed them into the now open corners. This gave them extra strength so that they don't collapse. If you have lots of extra glue, it is possible to 'fill' these spaces with hot glue, and that would give you a great support structure. I did not do this, the stuffed cardboard worked great.

Step 4: More Paint and Then Gluing the Last Pieces.

If you haven't already, paint the pink lines on all sides of the cube. You are essentially dividing each face with a pink painted line. This is going to take 3 coats as well. Luckily acrylic dries fast, usually in under 10 minutes.

Now, you have two options. Paint pink hearts on the now-white wooden disks and then glue them to the cube, or glue, then paint. Whichever you prefer.

Now you should have bright pink lines painted on all faces of the cube (I didn't paint the bottom).

Next, glue your white wooden heart disks in the center of each face. We're almost done!

The last pieces go between the corners. Cut a small rectangle out of cardboard and fold it over one of the edges. Next, cut the edges down until it is even with the pointy corners on the white companion cube corners (see image).

Next, cut a small curve out of the ends of the cardboard to that it completes the circle. Do this on both ends. Trace and cut out 12 of them.

Spray paint them white (3 coats, remember) and then glue them over the edges. Be careful on the bottom back edge. Do not glue your box shut!
You're finished!

Just put in your favorite brand of tissue and you're good to go.

It is important to note that I had to cut about a 1/4 inch off the top of the tissue box in order to get it to fit inside the companion cube.