It used (heavy) waxed paper "stencils". These were placed in a typewriter to create the original; the typewriter cutting through the paper. The stencil was wrapped around the drum of the (manual or electrical) machine, which forced ink out through the cut marks on the stencil. The paper had a surface texture (like bond paper), and the ink was black. It did not smell. You could use special knives to cut stencils by hand, but you couldn't really hand-write on them, because any loop would cut a hole, so you'd have a black blob. If you put the stencil on the drum wrong-side-out, your copies came out mirror-images.
The Gestetner corporation still makes and sells highly automated mimeo machines (externally similar to photocopiers), as the mimeo process can be less expensive than xerography for large print runs.
Mimeographs were used extensively in the production of fanzines in the middle 20th century, before photocopiers became widespread.
See also: duplicating machines.