Remove any existing hem with a seam ripper. This tool has a sharp metal edge that cuts through the threads to remove old stitches so you can make your new, improved ones.
Stand in front of a full-length mirror and turn the dress hem up until you reach your desired length. Place a few straight pins in the front of your dress to give you an idea about where you will hem your skirt.
Take off the dress and turn it inside out. Measure the length from where you placed your pin to the edge of the fabric. This will be the length of the hem you will ultimately sew -- and it’s important to ensure this hem is an equal length all the way around.
Use a fabric pencil and your ruler to mark all the way around the new hemline of the dress. Fold up the hem, following the pencil line, and place straight pins in the fabric about 1 inch apart. Iron this fold so it lies flat -- this makes it easier to sew and gives you the best idea of how the dress will actually look.
Put the dress back on again to double-check that this is the right length for you. If it is, you’re almost ready to sew.
Turn your dress inside out again and measure a new length from all the way around, this time measuring about an 1 inch away from the bottom of the dress. Turn this amount of the fabric under so you will have a finished edge on your new hem. This measurement will be where you actually sew the hem.
Use the topstitch function on your sewing machine to sew the hem along your newly marked line, if you have a machine. If you don’t have a sewing machine, you can achieve this by making a hem stitch. Thread your needle and tie a knot at the end. Put the needle in first on the fabric edge to be hemmed, about 1/8 of an inch down. Lift the needle and thread up, inserting the needle through the outside of the fabric visible to the outside of your clothing. Using as small a stitch as possible, put the needle back through both the fabric and the folded edge of the hem. Advance your needle and thread 1/4 inch ahead, then loop the needle and thread back through your hem and then the outside of your fabric until you have completed the hem. When you're done, the inner portion of your dress should look like small dashes all around your dress.