It starts with something small. New eye colour. Specific design on a T-shirt.
Before you know it, you are creating multi-layered, shiny outerwear in twelve different swatch colours.
I can't recommend playing around with modding more highly. There's a special kick you get out of seeing a sim walking around wearing a T-shirt that you drew yourself - something that was an idea in your head only days before. Even that is leaving out the more complicated modding that involves adding in new models for hair and such via Blender.