There are two main approaches to making monotypes, both of which involve making an image on the surface of a smooth, non-absorbent sheet of plastic. In the dark field monotype approach, ink is rolled over the plastic sheet then, using brushes, rags or Q-tips, selectively removed to create a subtractive image. The light field approach is to simply paint or draw ink onto the plastic sheet. As you can see from the process videos in my blog, I usually use a mixture of the two approaches, pushing and pulling the light in the image until I'm satisfied. I then place the plastic sheet on my printing press and a sheet of dampened paper over that again. The image is transferred to the paper under extreme pressure when the two are run through the press together.
Monotyping produces a unique print, or monotype. Although subsequent reprintings (called "ghost prints") are sometimes possible, they differ greatly from the first print and are generally considered inferior.