I took some pictures as I made the other skirt. It's not a tutorial, just a few step by step pictures that give an idea of the simple process.
First, find a sock. We have a lot of orphan socks around here, but these were a worn out pair.
The size of this ribbed men's Smart Wool sock worked well.
I turned the sock inside out and pulled the top part of the sock up over the doll's waist to make a mini skirt that just barely covered her bum. The bottom part of the sock became the hem of the skirt and I folded it up so I could hem it. It was really important to hem it on the doll so that I knew it would be able to stretch over her legs.
I'm not the most careful or patient seamstress, so here's what the hem looks like. It's okay, because I'll turn that skirt right-side-out and pull the part I hemmed up as the waist.
Here are the finished skirts on Grace and Katherine. They are also sporting custom t-shirts with felt designs. My daughter would probably narrate what they're doing in this way (because she watches a lot of YouTube videos about morning routines, etc.):
"Grace and Katherine are all ready for a day out. They're wearing adorable brown knit skirts and the most adorable custom t-shirts. Grace is wearing cute brown shoes and Katherine is wearing purple flats to match her tee. They've packed their tote bag and are ready to go."
The shirt on Katherine (on the right) was made using an American Girl Crafts kit. The cupcake tee on Grace is a t-shirt I bought on Amazon embellished with a cupcake made from a pattern in the same AG craft kit. The dogs are glued onto Katherine's shirt but the cupcake on Grace's shirt is affixed with velcro so my daughter can switch it out for other designs like the heart in the first picture of the post (and below on Grace's hand).