This a very useful tutorial on how to make halftone color separations for four color process printing in Adobe Photoshop.
These are just some simple tips and for further insight you can also check out this video.
For a very detailed process of CMYK color separation we recommend you check out this description of every step in the article from Graphic Design Stack Exchange, a question and answer site for Graphic Design professionals, students, and enthusiasts. It’s 100% free, no registration required and I am sure it will help you with other questions you might have regarding, printing graphic design tutorials and advice from professionals with experience.
CMYK color separation for printing