Growing an exposed root bonsai -- and it is done with many species, from pine to azalea -- is exactly like growing a root-over rock. In fact, a stone is often used around which the roots can be artistically arranged. It then is planted deep -- stone and all -- in a pot and allowed to grow for SEVERAL years. How long depends on the species, the climate in which it is grown, the fertilizing regime and a host of other factors, but more than 4 years is a safe estimate.
After 4 years have passed, you repot, exposing the top part of the roots. This is repeated for the next few years until all of the root is eventually exposed. Then, you can remove the rock for a free-standing exposed root, or keep it for a root-over-rock bonsai.
It Takes Patience.
Rarely, you can find a yamadori with long, thick, roots that can be dug and almost be naturally trainedas exposed root. This is NOT common, though.