STEP 1: Remove collar and sleeves. Most importantly, introducing - Emma!
My sweet Emma, like most cats, prefers to sit on projects while they are being worked on.
STEP 2: Use an old shirt that fits pretty well as a guide to estimate how much I need to take in the sides and how to angle the new sleeves. I turned the shirt inside out and used a marker to loosely outline the shape that I wanted for the sides and the sleeves. Then I sewed everything up and cut off the excess.
Optional: Use cat as paper weight in case shirts fly away