We made it up as we went. My thinking was it needed to be a quick, gratifying project, so I figured we'd need a chunky yarn. She wanted brown, and I just happened to have some brown Bernat Softee Chunky laying around - it's 100% acrylic, so they're good in the washer and dryer. I think it will soften up more after it's washed.
Using 6mm straight needles, she cast on 25 stitches, and she knit in garter-stitch until it stretched around her hand and wrist lengthwise - we didn't want them loose at all. Then she bound off all the stitches, and she made a second warmer exactly the same length.
I, then, folded them in half lengthwise and seamed an inch from the top, left about an inch and half for her thumb, and seamed the rest. She is very pleased with the result. In fact, before she could argue, I insisted she cast on for a smaller version for her best friend who is quite petite. When the girls are together, it looks like I have three daughters. It is fun to imagine - especially since it is just imagining. :)
I am under the impression that she would like to wear them on her first day of school. That's a good sign even though she keeps reminding me that she doesn't like knitting - not at all.