First, you need to assess the damage. Check for broken parts or worn-out components. Then, gather the necessary tools and replacement parts. It's not too complicated if you know what you're doing.
It's hard to say exactly. Maybe they had issues with the care they received, or there could have been some sort of misunderstanding or conflict. The possibilities vary depending on the cartoon's plot.
There are several techniques. One common one is using shading to create depth and dimension. You can also play with color gradients for a more realistic look.