Maxi Dress
This assumes you can sew that generic dress that everyone learns to sew at first. You know, with the lined bodice and the long skirt? Some call it the generidress.
You need:
Cloth -- Quilting cotton will do you fine.
Sewing supplies -- Needle, thread, sewing machine,
scissors, so forth.
Adult maxi dresses seem to have a specific formula: bodice, knee length skirt, then a ruffle that goes between the knee and the ankle. Kid maxi dresses don't seem to have a specific formula, so as long as we're getting a skirt and a ruffle on there we're good.
Either way, figure up how long you want your skirt, and how long you want your ruffle. The skirt should be as wide as your doll's waistline plus an inch and a half, and the ruffle should be twice as long.
Cut out any bodice you want, then cut the skirt and ruffle out according to whatever you figured out. Hem the two short edges of the skirt, and hem one long edge and two short edges of the ruffle.
Gather the ruffle so that it's the same width as the skirt. Stitch the unfinished edge of the ruffle to an unfinished edge of the skirt, right sides together.
Finish the back of your bodice if you didn't line it.
Now, stitch the other end of the skirt to the bodice, right sides together.
Everything's already finished up, but still, add velcro to finish.