A few years ago I knitted a scarf using stockinette and reverse stockinette and it had an undulated effect. Mine was just knitted straight across, not diagonal. I think for a diagonal, I would alternate the stockinette and reverse stockinette within the row and move the pattern over by one stitch in each row, eg., k5, p5. k5, p5 on the first row. The second row, p1, k5, p5, k5, p4, etc. I don't know how experienced you are, but if you are fairly new to knitting, I think this would be easier than increasing and deceasing at the ends and keeping in pattern.
Does this make sense?