Spot color is for logos, text and flat shapes: each solid color becomes one ink. It is the cleanest and cheapest to print, and most business artwork needs nothing else.
CMYK process rebuilds a full-color image from cyan, magenta, yellow and black dots. It works best for photographs printed on white or very light garments.
Simulated process uses a handful of spot inks, plus an underbase, to reproduce photo-real artwork on dark garments, where CMYK inks would disappear. Index separations do a similar job with square dots of solid color, which some presses hold more easily than fine halftones.
Not sure which one you need? Send the artwork and the garment color, and we recommend the method before any work starts.
Separation is only as good as the artwork behind it. If your file is a blurry JPG, we redraw it first as clean screen-print vector art, then separate it.