A Line Dress
This is quick and easy. Also, it's the basis for 90% of Kit's outfits!
Cloth -- Anything will do.
Sewing Supplies -- Needle, thread, scissors.
Bodice Pattern -- whatever you use for regular shirts and dresses will be awesome.
A line dresses are more of a cutting trick than a sewing trick. Measure your doll from her neckline to wherever you want the dress to fall, and add a half inch.
Fold your cloth. Place your bodice pattern on the edge. The length between the top of the neckline and the edge of the cloth should equal that measurement you just took. Leave space behind the pattern!
Now, cut your cloth, but when you get to the armpit, cut diagonally away from the bodice to the edge of the cloth. Here, have an MSPaint diagram:
Ta da! Unfold it to get the front of the dress.
Now, cut two backs like you would if you were making a shirt. You'll come up with three pieces.
Rock on, right? Finish it like a regular shirt.