I made a size 5-6 using this pattern here. It was really easy to follow with lots of photos to help along the way. There is a little bit of hand sewing involved, which isn't one of my strong points, but if I can do it anyone can.
It's the first time that I've sewn something like this and considering that, I'm pretty happy with the end result. I used fleece and flannelette from my stash and they make the vest extra warm and cozy.