I learned graphic art design in the day of "paste-up" when you either "shot" a picture of what you needed in a very large camera with a halftone screen on the glass or you "pasted" gradient dot pattern effects onto one of the paste up boards.... Yeah really, it wasn't that long ago. :)
I found an excellent tutorial on creating 4-color printing by using Adobe Photoshop and the tools it has available to create the 4 different screens needed. I loved the article, and the way it was written. This author knows creative, busy people and actually refuses to give you all the information til you've read the information you NEED to understand. I liked it, I learned a LOT so I thought I would pass on the link:
I am looking forward to trying out the technique and upscaling my client's product line - not to mention saving a lot of time converting the digital images into line art...that way I can get back to creating web pages and logos - which I love to do!