Well first you'd need photoshop or whatever advanced paint program is out there.
Then you need to have some sort of artistic talent in making sure the milk flows correctly without looking fake. So basically if you can find any images of spilt milk on a woman's breast that would be a good reference to use.
Then it's all about the brush you use, the color, and changing it around so that it doesn't look fake.
When you first put the white brush on the breast it's just going to look like a white line. But by erasing and making the line smaller or even making a seperate layer to put a filter over you can start making it a bit more realistic.
What I would do first, the easiest way is to make a new rectangle layer, adjust the hue/saturation, levels, and colors so that it looks like the color of milk on top of a breast.
Then I'd erase portions of the layer so that it wouldn't look like a rectangle and pretty much slowly start erasing areas so that it looks more like milk flowing. Then I'd mess with the opacity and see if there's anything I can do with the blending options to make it look more realistic.
But this is really all from an artistic point of view, it's really more complicated than that but that's a good place to start from.
More advanced artists would probably get a brush and just paint it on while using different tools like multiply, linear burn, etc. to make it look just right.
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