Carbon paper is an old school thing, as in during the era when typewriters were the main instrument for producing text. I will describe the oldest method of using carbon paper.
Imagine a sheet of white typing paper, on which you will type. Of course, the typewriter will strike a ribbon first and then hit the paper, transferring a bit of ink in the shape of the letter.
However, before you put the typing paper in the typewriter, you grab two things: a piece of carbon paper and another sheet of white typing paper. You layer everything like this:
top ---> typing paper #1
middle ---> carbon paper
bottom ---> typing paper #2
You now put all three into the typewriter roll together and start typing. The physical force of each key striking the top piece of paper transfers a bit of ink from the back of the carbon paper onto the second sheet of paper.
You now have made two copies of the same document.
Old school, baby!!