well each seperation is refered to as a plate, in 4 color process (CMYK) the inks which are used are far more transparent. So this is OK for printing on a white garment, however when you print on a colored shirt you wont get the colors right becaue of the color of the shirt showing througfh the opacity of the ink. So you will have to add an additional place (mostly a white) to print under all these other colors. Also you will have subtract some of the white plate around the edges so that the white doesn't show. On some designs there are different areas that will knockout to the shirt color, and thats were it gerts tricky doing that white plate (sometimes refered to as flash plate).
LJ, What your asking is a tall order. I believe Scott F. sell dvds on this subject at U.S. Screen Print and Inkjet Technology. you might check that out. Separating in photoshop is not easy and takes quite some time to learn. When you dont know much about the software to begin with , well just makes it that much harder to lear/teach.
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