How would I make a texture for a model without one?

How would I make a texture for a custom model?

Think of it as taking the way every face of the model should look, and painting it on a flat surface.

What AntiSaint said. Also, I don’t know what modelling program you’re using, but most will have “UV Unwrapping”. Basically what it does is take the polygons you have selected on your model and flatten them out for use with texturing. Look up tutorials on how to use UV Unwrapping for your modelling program.