how do i buy things from the hong kong store?

I did this once - bought music from the Hong Kong iTunes store (because the artist was not available on the US store) and then switched back so that i could buy from the US iTunes store again. Except that I have no idea how I did it, and I can find no instructions as to how you can authorized your account to go to any country's store other than your own default store.

Is there an apple instruction on how you do this? Can anybody tell me. There is a song in Chinese from YouTube that I love, but there are no obvious links how to buy it. I suspect the artist (A-Lin) must have many songs in the Hong Kong store. How do I get there again?