I am drawing it in Paint Tool SAI using a tablet and then I will do most of the colouring in Photoshop. If by background you mean the isometric grid, I made that in photoshop first by drawing two horizontal lines on a transparent background, duplicating the layer, rotating one layer 30 degrees clockwise, the other in the opposite direction, merging the layers, cropping to the "diamond" you end up in the middle and then going to edit -> define pattern.

Alternatively there are a few grids around the forum somewhere!