What you'll need:
1. Poster board (once you get this down you can use a thicker chip board for more durability)
2. Exacto Knife
4. Pen/Pencil/Writing instrument of some kind
5. Cutting board
6. Bone Folder (Scorer, whatever you want to call it)
7. Double Stick Tape (I know, I know...I have glue pictured but that was before I remembered that glue sucks and double stick tape is the shit and I'm lazy and didn't want to retake the pic. I know some people don't like it, but I love it, let's put that shit on everything...but I digress)
This here is your handy dandy diagram of how to cut this out. I did a 5x5 box, so you will want six 5x5 boxes altogether. Leave a 1/2 inch tab on the 5 outermost boxes. It's really hard to explain, but basically draw the diagram above on your poster board.
Now the tricky part. Using your Exacto and ruler cut out your box, but make sure to just cut the outermost lines. The rectangles that were made for tabs will also need to be cut in the inner corner so that they can fold in. Again, reference diagram. When cut out, it should look something like this:
Then using your bone folder and ruler, score all the areas that will be folded. Scoring just makes it easier to fold and makes sure that you have a straight line.
Once scored, apply double stick tape to the tabs on the box. Stick the tabs to the inside of the side kitty corner to it. It's pretty self explanatory once you get to this point...you're making a box, soooo make it look like a box. Try to align the edges as best as possible, the box will work best if everything is even.
Fold the top down, but do not tape that side or else you just have a cube, not a box. But that's it! Once you get this down you can move on to more advanced boxes and shapes. If you are using this for a mock up, you would then print out your design, preferably in the same shape as the the diagram and wrap that around it. Sorry I'm not great at explaining what to do, but it's not too tough if you have the correct shape to cut out. Try it! Lemme know how it worked!