If you draw it, it'll make more sense. This is one place where it's hard to beat old-fashioned graph paper.
When you finish the first row that you just knit:
Increase at the center front for eight rows, then on the eleventh and fourteenth rows (I think--check my math.) That's going to make a slanted edge that starts out steep and becomes more gradual.
The other edge, where the side seam will be, needs increases too, but not quite in the same spot, so you only add a stitch at the beginning of the 8th row and the 14th. The taper on that side will be very small and gradual.