...is a question with many good answers. But I wanted to try it anyway.
After searching the internet for inspiration, I settled on the Burda 9501 girls' coat pattern. I was drawn to the simple, classic lines and unfussy details. This was my first time both sewing a coat and using a Burda pattern so supplementary tutorial reading was definitely in order.
I followed the pattern instructions - such as they were - for the most part, but decided to sew the entire lining in by machine by following this tutorial for bagging a lining and managed a pretty fabulous jump hem thanks to the excellent instructions. I recounted my triumph to my husband - previously blissfully unaware of what a jump hem was - repeatedly and enthusiastically.
I was already leaning toward substituting a hood for the collar, and after reading more than one complaint on the collar size and instructions for this pattern, my mind was made up. I wanted a three-piece hood and went through about eighty bazillion yards of tracing paper trying to draft one before throwing my french curve down and buying the School Days coat pattern from Oliver and S. to use that hood. (I wanted to own this pattern anyway to make a coat for my son, so it was pretty much a win-win.)
Naomi chose the fabric from a highly-edited-by-me selection. It's the Arrow Canvas in Mint Metallic from Cotton and Steel. We both love the bright colors and playful design. She calls it her Merida coat after the arrow-shooting heroine from the movie Brave.
The lining is a flannel print I picked up at Joann's.
All in all, it was a pretty painless and educational experience, and I'd definitely make this pattern again. And now, because he's so cute but hasn't gotten a coat yet: