I think Smorbs gives an easy and quick way of doing it.
I would only add that you need to use your seat too, it kind of helps them figure out which way you mean for them to move.
I don't mean to lean and get off balance, but to just shift your weight a little to the inside to encourage movement in that direction along with the rein cue.
Eventually you can get your turns just from your seat, with reins there just in case.