A callout sheet is a condept design from the concept designer or art director telling you what kind of look should your final character look like. A more detailed call out sheet can also be done by you to get a buy off from them before proceeding to start the texture painting process. This can then be used too project straight on to the 3d geometry as a base map to start painting on before proceeding to clean up the seams and the stretching from the projection.
Texture painting can be boiled down to 2 basic methodology. Creating a texture map from scratch with nothing but different brushes and then photograph manipulation. These of course are not mutually exclusive to each other.
It is more about choosing which method to use to and how much these 2 methods overlap over the creating of an issue that is the challenge. Predominately, the ability to paint is essential. I have seen many painters that are over-reliant to photo manipulation whereas a few simple brush strokes here and there might just do the job.
A color corrected photograph might be a ideal place to start. And the details can be be added on top.
There is a certain feel of liberation that a brush gives you, the confidence that if the painting is screwed up, u can just paint over it. Again, how this is applied needs to be defined on which layers are to be done.
The transition from one part of the body to another is based on what kind of character it is. Amphibian or mammals etc.. Looking at different references are important to understanding the different types of surface materials and making an informed decision on the what is a suitable and aesthetically pleasing surface material for your asset.
What I tell myself everyday.
To all the people watching, I can never ever thank you enough for the kindness to me, I'll think about it for the rest of my life. All I ask is one thing, and this is.. I'm asking this particularily of young people that watch: Please do not be cynical. I hate cynicism - for the record it's my least favorite quality, it doesn't lead anywhere. Nobody in life gets exactly what they thought they were going to get. But if you work really hard and you're kind, amazing things will happen. I'm telling you, amazing things will happen." - Conan 'O'Brien