You set the number of shards you want and click "rebalance" (CLI tools are available too, of course). The right data is replicated to the right nodes without any additional effort. It's demoed in the screencast video if you'd like to see (http://www.rethinkdb.com/screencast/).
I was actually a little worried when I read that everything can be handled from the Admin GUI interface. Knowing that there are CLI tools available makes me a little more comfortable.
For some reason reading through steps like "click here" makes me less comfortable than "type this".
