Lots and lots of practice... and a touch of laziness
As for the actual method, I sketched it out, and then did some scribbly lineart overtop. I added another layer underneath, and made a solid colour fill to shape out his silhouette. Then, with various textured brushes with opacity settings on, I added the colours, light and shade.
The background is mostly gradients that I painted over with fuzzy or scratchy brushes.
In some places, I recoloured or removed the lineart so it didn't clash with the surrounding colours.