While converting to CYMK is as simple as selecting the CYMK mode from the mode menu. There is a little more to it then that. If you are going to work in cymk mode you need to talk to your printer or service provider first, There is information you will need to get from you printer. This information will need plugged into your CYMK set-up. I can't stress enough see! your printer first. It will save you money and pain later. I assume you are planning your work for reproduced on a offset or gravure press hence the question? If you are just going out to you home inkjet printer leave your image RGB as the drivers for most inkjet just convert your CYMK back to RGB and user there own setup for converting to CYMK. James Stuart Johnson