nikschaf, i kind of went off of a baby onesie. i tried to see how much they overlapped the shoulders, and adjust for the "scale" of an adult shirt.... you NEED extra fabric at the shoulders, so this works best with a shirt from scratch, or a REALLY huge shirt. if you try to lap the shoulders on something that fits correctly, it'll end up too short
i can try to work on a tutorial if you'd like, but the gist is adding about 5 inches or so to the shoulders (front and back) and lapping them. i find it easier to make adjustments to the neckline after i get the sleeves right.