The RDL is a versatile exercise that can be done in a variety of positions, environments, and loaded with a number of different modes. I prefer to start people off with a dumbbell variation because it allows the individuals to manipulate the load out of their way when they extend their hips at the top of the movement.
To properly perform an RDL you want to begin standing upright with your shoulders, hips, and knees in a straight line with the spine in a neutral position. You want to maintain the neutral spine position throughout the entire exercise. Begin the movement my slightly flexing the knees once this position is assumed do not flex the knee any further. The rest of the exercise's motion should come from the flexion of the hips, think about driving your butt towards the wall behind you. As your hips flex backwards the dumbbells should migrate from the sides of your legs to the front of your body remaining close to your legs as you flex your hips. When the bottom position is reached you should feel a tight stretch in your hamstrings. Once you have maxed out your hip flexion range of motion. Initiate the the concentric part of the movement by extending the hips forward and squeezing the glutes. Remember to extend the knees as well and finish standing upright once again. This concentric motion should appear as if you are humping the air in front of you.
I recommend using this exercise once a week atleast focusing on movement quality over load. Once your proficiency has increased then begin placing increased loads throughout the movement. Variations of this exercise are numerous so if this doesn't quite work for you chances are there is a variation out there that better suits your physiology.