Always start the illustration with a rough scribble. This helps to get the idea on paper faster. While you do that, it's advisable to check if it's working in the layout or not. After that's finalised, make a fine line drawing by putting all detailing in place. After that comes the lighting. Give a monochromatic tone on the entire sketch and decide the light source and the reflection. This gives a fair idea about the final look.
Next, create different objects in separate layers. Once all the objects have been created, then give the basic shades in respective colours for each layer. This helps understand the colour tone for the illustration in the required light. Here, an 80-volt bulb effect is created in the shady bathroom. One thing that needs to be aware of is that the highlight cannot be pure white. The entire tone has to be yellowish.
Now, select a soft brush to work in most of the places. Notice, a rough brush is also used in certain spaces that help colour filling to be faster.
Once the direction of light is figured, we can move forward in details. Here, the tiles have been put in perspective after which the lights and reflections have followed. It is important to note that light plays tricks with different surfaces. Here, the bathroom tiles must reflect the light. If you are unable to judge the light, then it's better to visit the actual site where you can see the reflection and tone in the required direction.
The next stage is to get the expression right. Notice how the shadow formation on the expression of the character is what gives the entire look. Here, the reflection is monotonous. If this were an outdoor scene, the shadow light would have been in a bluish tone. However, that is not the case in this illustration. After getting the light, shadow and reflection right, the next step is to focus on further detailing. This includes creating facial hair. Notice how close the bulb is to the head and so a glare effect is created on top of objects below the light source. Hence, a yellowish tone is made to overlap the character.
One can add as many elements to create detail in the picture. Here, crumpled pieces of paper are added to the story. Notice how the light direction changes on each paper. If you have trouble understanding light and shadow, then it's always better to get a reference. Take pictures from a camera of the objects for reference.
The final stage is the completion of the illustration. One very important thing to keep in mind is to always keep saving your Photoshop le multiple times. You never know when the le might get corrupted and you might end up banging your head on the monitor.