The dumbbell Romanian Deadlift is performed much the same way as the barbell version. Both variations are great for hamstrings and overall posterior chain development.
While you can start with the dumbbells at your sides, be sure to replicate a holding a barbell as you perform the lift, keeping them in front.
Start the exercise with the knees slightly bent. Bring your hips back as far as possible. A good cue is to think about putting as much distance between your hips and your head as possible.
The main thing to aim for is a feeling of stretch in the hamstrings. If you're not feeling that or want to really recruit the hams, place your toes on 5 or 10-pound weight plates.
This trick based on one simple biomechanical principle: the muscle being stretched the most is the one being recruited the most. With that in mind, a very simple way to get more hamstrings growth from this movement is to elevate the front of your feet by half an inch to one inch. You also shift the balance point to the back of the feet which enhances the recruitment of the posterior chain muscles.