How do they do the art for minecraft?

Usually people do it in Creative mode first before doing it in their SMP(Survival Multi-Player). I suggest starting with 8-bit images and for some it is known as Pixel Arts. Because they are very very easy to do because you can count how many blocks each segments are. Here is an example and very easy to do. You can do it on the ground or make it stand. Building houses and other kinds of buildings in minecraft can be called a work of art too. If you have a creative design on houses/buildings or whatever you have in mind you can make there as an art. After a while when you progress in your work you'd be able to do something real something big. AKA PROJETS

Oh and also, texture packs makes them even more artsy.
1. Megaman
2. Simple House
3. Medium House
4. Advanced
5. Project 1
6. Project 2