First, you need to construct the hand correctly.
2. Make sure you're using a quality reference.
3. Know and prepare your art supplies
4. Always start with a light initial sketch, focusing on the largest shapes first
5. Keep in mind that in realism, there are no visible lines
6. Create gradual, smooth transitions between your different values. (In order for your drawing to really pop out and transmit a sense of realistic three-dimensionality, you have to develop a huge range of values throughout your piece. There have to be very light areas (which will appear almost white at the end), there have to be very dark areas (which will appear almost black at the end) and there have to be a ton of mid-values in between.)
Explanation: Hope this helps