I finally finished my other jacket! I used one of my 3 year old's shirts for reference, but cut my pieces about 1 inch larger because I wanted it to be able to fit over other clothes. I cut the bottom about 2 inches below the armpit thinking I was going to make a jacket with an empire waist, but after trying it on halfway through, my daughter and I decided it should be a bolero.
I folded my material in half to cut it out to make sure it would be symmetrical. (note: I cut out two of these)
I took one and folded it in half to cut the front open and lower the neckline.
I traced the sleeve of the existing shirt to get this cutting line too, but I made sure and lined it up with the hem of the shirt I was reusing.
No pictures of the rest of the process, but it was pretty standard. I basically did something kind of like this link from Crafterhours, but there is no ribbing to worry about. Then I made bias tape like this tutorial from Craftiness is not Optional, and sewed it all the way around. Finally, I made a rectangle with 2 button wholes and sewed buttons on both sides of the front opening.
My 1 year old just won't stay away with her books, and you don't want to see the 3 year old's face...well, maybe I don't want you to see her face.