Plating is knitting with two yarns together in one feeder and this creates a 2-color pattern in a structure without creating a jacquard. Make sure the two yarns you're using are of a finer yarn count, so that together they are of a yarn count that knits best on Kniterate (i.e. between Nm6 and Nm8). Find more information on yarn counts here.
One of the yarns goes through the knitting hole and the other through the plating slit, making sure one yarn shows in the front of the knitting while the other shows in the back. The yarns will show through a bit on the other side, creating a mixed colour effect. When both holes in a yarn feeder are threaded for plating, they knit at the same time as the carriage moves left and right. A plated rib will show vertical columns of both colors, and you can create interesting fabrics when you play around with it a bit.