It has an adorable, across the chest style. True to my lazy fashion, I chose to work with fleece because it's easy and you don't need to worry about finishing seams.
First I cut out a pattern from parchment paper. I measured the distance from my shoulder to my hip for the length (and added two inches). I made sure my parchment paper was that long.
Next, I decided how wide I wanted it to be. I picked half the length, giving me a 24 inch by 12 inch rectangle. I tapered the side to give it that elongated look, then mirrored the cut on the three other sides.
I traced this onto white fleece for the back/bottom of the bag.
I cut out two of these.
I also cut out a long strip of green fleece, the length from my shoulder, down my chest, to my hip.
Next, layering. Place the bottom piece FACE UP. Next, lay down the green strip for the handle, aligning both of the ends with the tapered ends of the bag. The green strap will be completely inside the bag. (I'm sorry I didn't take pictures of this step!)
Then, place down the top flap that will be facing out. It's RIGHT SIDE needs to be facing down, laying atop the bottom piece and the green strap.
Finally, lay down the second inside flap. None of its seams will be seen, so it doesn't matter how the edges look.
Sew completely around the bag. At the tapered edges, you will be sewing through FOUR layers (bottom, strap, top one, top two). My machine did this without problem. Usually it gets picky when I send fleece through it, but it gave me no trouble this time.
Once it's all sewn, flip it inside out and it's perfect! All the seams are nice and neat and pretty!
Now all you have to do is sew on the button.
Arrietty's bag has a big, red button. I bought one at Hobby Lobby for about $3. I sewed it on the bottom of the top layers, then sliced a buttonhole on the top of the bottom layers.
And now pair it with the dress and you have this: