Hold the DB in the same hand as the leg that will be stationary.
With the DB against your thigh, retract your shoulder blades and maintain a braced core and neutral spine so your chest sticks out. This will be your starting stance.
Bend/hinge slowly at the hips maintaining flex in your knee, lower the weight (and your torso) straight down your legs until you feel the maximum stretch in your hamstrings.
The bottom position will be roughly knee-level.
Upon reaching the max stretch, begin lifting the weight back up via hamstring and glute utilization until you reach the starting position.
*Tips*
Keep the DB close to your center-of-gravity.
Keep a slight flex in both knees at all times.
Imagine you are raising your back heel towards the sky.
Don't let your low back arch excessively.
Ignore the notion to lower the DB as low as possible.