This would depend entirely on your skill level as well as what you mean by human figure. A 2D model is simple enough - just have someone lie down on a large sheet of cardboard and trace around them, cut out the silhouette and voila.
3D projects have lots of variety in their execution. It is a typical exercise in most art colleges to have students make 3D cardboard human models. Viewing a college art gallery or touring their workshops first hand is a great way to see their methods, techniques and results.
When working with cardboard, a X-acto knife is your best friend. That and a glue gun, depending on your style. Some people tear away the paper on one side in order to expose the paper ribs inside the cardboard. This makes the cardboard pliable. With the ribs exposed the cardboard can now be rolled into limbs, fingers, etc. Some artists prefer to make cuboid figures and so only score one side of the cardboard with their knife where they wish it to bend. This makes for cube-like limbs and features. Some like it difficult and cut each individual piece and reassemble it onto a frame or use styrofoam blocks as glue points to connect the panels to each other.
You will have to experiment and decide what style and look is the one for you and your project. If you need lots of cardboard you can request it by going to companies/restaurants/grocery stores that have a lot of incoming shipping - usually they just break down the boxes and ship them out to the trash or for recycling if not reuse. Make sure never to take cardboard that is wet, has been wet, or has organic mess on it (such as fruit or vegetable boxes) - they are not good material.
