Put on the dress and mark the desired hem length with pins or tailor’s chalk
Remove the dress and measure the hem evenly all around
Add seam allowance if needed and cut off excess fabric
Fold the raw edge up to the marked hemline
Press the fold with an iron
Fold again if using a double-fold hem and press
Pin the hem in place
Sew the hem by hand or with a sewing machine
Remove the pins
Press the finished hem again