EDIT (2018): I had posted this instructable to inspire you to think outside the box on what is possible to achieve with just cardboard. I do not provide any blueprints or step by step guide to a specific build beyond sketches. I'll admit this isn't my best project but I'll do my best to turn it into something a little better and provide a little bit more information. I'll have to retroactively go through and take process pictures so it may take a little longer before pictures accompany the steps.
You can make anything out of cardboard with a bit of experimentation and practice. For this project I taught myself how to create a scale replica of an object. All I used was a box cutter, hot glue, and lots of large sheets of cardboard.
Step 1: Getting Your Materials
For whatever you are planning on building out of cardboard, your first step is to get large sheets of flat and clean cardboard.
I went to Sam's Club (any bulk shopping retailers will do) for most of the cardboard; they have large, flat, clean sheets of between their pallets that they let you take for free. You should be able to walk out with a cart full of cardboard but it doesn't hurt to ask or let someone know what you are doing. So far in my experience they are more than happy for you to take away what they consider to be trash. The paper products section is the best place to look (toilet paper, tissues, paper plates, etc). Those products are light enough they don't damage or dent the cardboard sheets.
I have found that Sam's Club pallets also have different qualities of cardboard so look for both thinner, more flexible sheets for curves and thicker,firmer cardboard for clean cuts and large sides.
For a large build, just plan on finding mountains of cardboard wherever you can. The more you have on hand the easier the build will go in case you need to make changes and re-cuts.
Step 2: Materials and Techniques
It's important to keep your blade of your box cutter new and sharp to keep the edges crisp and clean. Cardboard dulls blades incredibly quickly so be sure to have a supply on hand before you begin a large sculpture. A dull blade will catch on the cardboard causing rips and incomplete cuts. That's incredibly frustrating when you are trying to make a nice line that's 3 ft long!
Hot glue gun:
I have a love hate relationship with hot glue guns. They are a quick glue and it works incredibly with cardboard. However, I've never managed to avoid burning myself on a large project like this when you find yourself upside down in the head of a hammer. Like the box cutter blades, you'll need lots and lots of glue sticks on hand. Building something upwards of 7 ft tall uses a lot of glue.
It makes sense that you would need a straight edge to help you cut you lines and shapes. Its good to have a couple of different sized ones so you can accommodate long cuts and smaller ones without too much difficulty.
One technique I used to get nice curves was rolling the cardboard with the grain of the corrugation. Keep that in mind when drawing out your designs on the cardboard before you cut if you plan on having smooth curves. If you look at my hammer example, you'll see I used this technique for the neck of the hammer head. I usually just roll the card board over itself until its durable and flexible. Then I can glue the curve into the position I want in the final assembly.
Sometimes instead of cutting the cardboard you'll want to bend it. I recommend you have a long straight edge or a ruler around to help get a tight bend. This is especially important if you are going against the corrugated "grain" of the cardboard or across it at an angle. The corrugation will buckle or fold haphazardly unless you can indent and bend it with a ruler.
Take your time cutting cardboard. If its a thicker cardboard then it usually pays off to cut it out in two passes instead of one. The first pass establishes your line for you and the second pass will get it cut through. If you try to cut through in one pass, the brute force you apply usually makes the cuts more jagged and erratic.
Step 3: Planning Your Build
A cardboard build can be made up of cuts and bends. Its best to design with a good mix of cuts and bends. Cuts are far more labor intensive and your structure will be stronger and more supported with bends. However, bends usually require larger sheets of cardboard where cuts allow you to take from multiple pieces of cardboard.
In the photos you can see the sketches and measurements I made before I started cutting. I measured all the main points of a hammer I had and then scaled it up so all the proportions would be accurate and look right. A little planning really helped things go more smoothly as I worked on it, I still had to make some adaptions as I worked on it though. So be flexible! I had originally designed a bunch of tabs to hold my pieces together but that ended up being completely unnecessary as the hot glue did a fine job holding together at just the edges.
It'll help if you simplify your object a bit too. The handle of a hammer, in my case, is very curvy and ergonomic. I had planned on replicating that but it wasn't worth wrestling the cardboard to behave like a injection molded rubber handle. My handle was simplified but it still looks like a hammer!
Step 4: Build Something Larger Than Life!
I love the fact that I made it tall enough to stand under, it really makes it feel surreal! It also barely weighs anything! Get out there and start having fun with cardboard! Its cheap, lightweight, and easy to play with. I hope I inspired some creativity!