I find curling the ball with the outside of my foot a lot easier than doing it with the inside... strange, no?
There are a few different ways I bend the ball with the outside of my foot, depending on whether it is stationary, rolling, or in the air.
Stationary) Drive the ball as you would to shoot, but cut across the ball more than normal. For a right footed player, that means moving your foot more to the left than normal as you strike.
Moving) Here I hit the ball with the outside side of my foot... almost down to the heel. You can get a tremendous amount of curl here simply by doing that simple action... but I may only be able to do it due to a combination of practice, bendy legs, and screwed ligaments!
In the Air) As with stationary, shoot fairly normally, but hitting the ball on the left side (for a right footed player) and cutting across.
I'm sorry if these explanations aren't very good... would be a lot easier to show you in practice. Hope it helps anyway!