First I took the sleeves off with seam rippers. Then I tried it on and put pins in the sides at my bust and waist to mark where to cut. I drew a curve (with silver sharpie on the right side of the fabric -- not recommended!) to connect the points and then cut.
I hemmed the sleeves to the point where the side seam would start, then stitched the seam. I had to make some adjustments for a nice flare, but I think it turned out surprisingly well!