THE LETTERPRESS PROCESS
If you can design it on your computer it can be printed letterpress. Letterpress printing is essentially the printing of a series of spot colors, which is to say it is not a four color process. Each color is a separate print run and the ink color for each run is mixed to match the PMS (uncoated) color that you specify. You can have as many colors as you want but, again, each color will require its own run.
The impression that is made in the paper depends on a number of things: The thickness of the paper (the thicker the paper, the more impression can be achieved). The hardness of the paper (the softer the paper the deeper the impression). The size of the type (the smaller and finer the type the more quickly it will distort under pressure). The density of the image (the more lines of type or art that there are the less impression they will make). For this last one think of walking across a field of snow wearing snow shoes. Now walk across that same field in a pair of high heels. Which foot wear leaves a deeper impression in the snow?
Halftones can be used though they will not leave any impression, or bite, in the paper. They work best with papers that are very smooth. Any texture that the paper has will cause the screen to fill in and the image will become muddy. Do not let your screens get too fine. A 120 line screen works well.
Almost all papers work well for letterpress printing. In fact, I have only ever had one paper specified in 20 years of printing that did not work with the letterpress process. And I chose that paper myself. Oh well, live and learn.
I print most of the jobs that I get using photopolymer plates. Over the centuries wood, lead, copper. zinc, magnesium and other materials have been used to transfer the ink to the paper in letterpress printing. Photopolymer, the latest material in this long list, is ideally suited for forming the bridge between your computer and the letterpress process, And it is the most ecologically friendly material that has ever been put to this use.