Pinning dresses
Get someone else to pin you. Chopping it off evenly just wont work. Each person has different posture, and different body type that will make an even hem crooked. For example a pregnant lady is rounder in the front, someone else may be rounder in the back. So an "even" hem would be shorter in the front/back respectively for each person.
SO. After you choose your length measure from the floor up. pinning all the way around the dress at the desired distance. I measure at 19.5 inched from the floor.
Iron & Cut
Fold at the pins and iron around the fullness of the skirt right on top of the pins. This ironed crease is your Finishing line. Then cut away extra cloth leaving a few inches for the hem. DO NOT CUT ON THE FINISHING LINE
Sew
Following the line from the ironed finish line, pull the cloth back 1/8 inch. Sew at the edge, 1/8 inch from the finish line. (The pin is where the ironed finish line was)
Cut
Cut very close to this stitching line.
Fold this narrow hem over 1/8 inch and sew again on top of first stitches.
Admire your work. Say things like huzzah.