First mark the hem. Obviously this would be done while the dress is on the girl that will wear it!
Next mark were the finished hem will be and take out the pins holding up the hem. You can see that I mark it with pins.
Slide a piece of paper under the edge of the curve, trace around the original hem and cut the paper along the line you just drew. Now you have a template for the new hem's curve.
Place the template on the dress, line the edge up with the pins and mark the rest of the curve--again, I just use pins.
Now you are all set to hem the dress so that it looks just like it did before it was shortened
If you are doing a tiny rolled hem like this one, don't forget to unpick the old hem several inches above where the new hem will start so that it will be a smooth transition.
Here it is--nice and curvy!