Finished this jacket this morning, made for daughter, as she wanted to wear over one of her top. Hope it comes useful to her.
For this, according to the length needed plus, the back together, make chain, then see how the crazy shell is made, once the breadth from neck to shoulder is reached, you decide how much you need for arm-hole, then in those stitches continue the pattern till desired length of the sleeves. You need to make 2 like this, as it is joined in the centre at the back, then join for sleeves and sides. Then make whatever edgings you like.