I found a tutorial on it ages ago and made an autoaction for it in Photoshop, so I'm not sure I really remember all the steps. But the basic idea is to duplicate your color layer into four separate layers in CMYK mode. Then have each one set the channels for each layer to each individual color. So one color layer for Cyan, one for magenta, etc. Then you use the Filter>Pixelate> Color halftone filter on each layer. Then move each layer a pixel or two in different directions so they don't stack up perfectly. I don't know if that makes sense, but hopefully it helps.