This works will with most variagated yarns.
YOu want a multiple of 4 plus 3 stitches to give width of scarf in the gauge you're knitting in. (You might have to swatch)
Knit five garter rows. Then every row is:
k3, *YO, k2tog, k2; repeat across.
finish with the same number of garter stitch rows you started with and bind off.