Eat cereal? Get clothes as a gift? Wind up with those thin cardstock boxes from somewhere else? Own a utility knife or just a loose razor blade? Well then YOU yes YOU can cut a stencil. It really is pretty easy.
Step 1: Get an image
First find an image you like. A quick web search for stencils should give you something cool to work with. Here we will use a Biohazard symbol because it is simple and I am lazy
Step 2: Prepare materials
If you are starting with boxes of some sort you will want to cut the sides off. It is best to set yourself up with the biggest side to cut from.
If you are using a found image (or making a copy of someone else's stencil) you will want a black and white copy of that image and a few glue sticks. Flip the image blank side up and smear some glue on it, you want to cover as much of the surface as you can with a thin layer of glue. There are also spray adhesives for things like this, but they are not available at the local dollar store.
Step 3: Sharp toys
Grab your utility knife, x-acto, or loose razor blades (my favorite for finer details) and start cutting. Don't be shy. You are turning trash into art/tools and If you make a mistake scotch tape is your friend. If you care at all about the table you are cutting on or just want to increase the life of your blades put a piece of corrugated cardboard under your work.
Step 4: Rattle can rewards
After you have cut your image out, grab the nearest can of spray paint and give it a go. Chances are once you do this for the first time you will never look at cardboard as trash again. If you have some strange desire to put whatever your image is in an unauthorized place It might be wise to spray a copy or two onto cardstock, just in case you.....ummm..... loose the original.