Steps:
1. Start with a sketch
It provides for quick iteration of ideas without committing to the long process of painting.
2. Establish perspective and set the mood
You should have some perspective guides before you paint anything.
3. Choose your brushes
Start using Photoshop’s Brush tool and Brush panel
4. Build the foundations
Start with simple shapes and silhouettes.
5. Turn on the lights
Define your main light source.
6. Find the details
You should find the details in each layer of the composition with Photoshop’s clipping layers.
7. Polish and distribute details
Add some details if you find something missing.