They breed with 0's and 1's, just like any other Pokemon.
*ahem* I would reckon that they breed with each other and non-Normal types through your standard processes. Though, I'm sure that if Ghosts can hit each other, they are then solid to each other and thus can easily breed with each other. As for how Ghosts breed with Normal types, though, I would say that these Pokemon can indeed deem each other as solid when the Ghost so pleases. Don't ask how it opts to make itself more felt when it gets into the daycare center, but...it does. Don't ask questions about it, it just does. 8D