Excuse the sad face. Kewpie likes the shirt, just not the photographer. She's two.
First of all, I cut the t-shirt off completely an inch under the arms.
Then I took a t-shirt that fit her well, folded it in half, poked the sleeves in, and traced the body.
You could also trace a sleeve, but I discovered that I didn't need them..
Then I placed the pattern on the top of the t-shirt, lining it up with the top seams and neckline (I was still going to use those) and traced it with soap. Then I cut around the arms and down. (Sorry, forgot to photo that.)
I cut about an inch off the sleeve at the part where it meets the shirt to make it shorter. Then I pinned and sewed the sleeves back in, but because the body was smaller now, the sleeves were too wide. That made for a cute little pucker in the top of the sleeve.
Then I took the bottom part of the shirt, gathered it, and reattached it to the now smaller shirt.
Last of all, I tried it on Kewpie, and she gave her approval, whew!
I would say it took me about 30-45 minutes.
Now to ponder on how to embellish it. A shamrock on the front?