I folded the shirt in half matching all seams to be symmetrical. then I cut it down the middle along the fold.
I don't have pictures for this part, but I pinned one shortened sleeve into the arm hole with right sides together. (If you're following along, remember that there is a top and bottom to the sleeve because of how it is cut with the slight curve.) There was hole under the sleeve since the new sleeve is smaller. Once I got the sleeve pinned, I sewed up the underarm hole and tried to taper it out a little. Then I sewed on the arm.
Phase 1 is here.