How would I texture a character in this style?

How would I texture a character in this style?

How would I got about texturing low poly models like this?: 

 

Everything in one diffuse map, so no PBR workflow.

 

Can someone give a list of topics/concepts/techniques I need to learn in order to create detailed textures like this for low poly characters?

 

Also what software would make the most sense for this task.

 

I already have modelled a low poly character so my current task is to texture it.

 

If you could also explain how you would go about texturing this character like this that would be great.

 

Thank you :)