As I like to extol, I don’t really know how to do alterations. But, a girl’s gotta do what a girl’s gotta do. The back yoke on the bottom is the original. I marked an inch off the back and graded to the hipline. The diagonal purple line is the new seam line.
Because of the taper, the top of the yoke was now going to dip. So I added about a 1/4 inch to the very top. If you look at the first pattern piece, you can see in the left hand upper corner that little addition.
How do I know all this? Look at the darts in your pants and tops. You’ll see that they have a little extra on top. So, between some educated guess work (and cutting out a second muslin yoke) I sorted it out.
Hope that all works out, eh?