Apply your lighter shade (50% Gray) in the direction of the knit all over the entire garment.
Apply your medium shade (75% Gray) in the direction of the knit. Make sure to leave some of the lighter shade as the highlight.
Apply your black marker in the direction of the knit. Make sure to apply the color in stripes to imply the pattern of the knit rib.
With your white Prismacolor pencil, redraw the seams an hight light some of the knit rib. For a variegated rib knit, alternate the rib stripes from thick to thin.