It's really really really easy. First remove the grip, which can be kind of difficult; jam a flathead in there and squirt some WD-40 if you need to and work at it until it comes off. Don't worry, you won't break it. Once it's off you'll be able to examine the metal throttle bar and how it pulls the throttle piece. It's really very simple, there are two lines cut out in the bar. One of them controls how much the bar will turn. This is the one you want to make longer. You can even use a circular file if you haven't got a dremel.
Because you can also adjust the cable at the carb, you can be generous with how much you file out, just be careful not to take so much out that the throttle piece (the one that holds the cable) won't come so far that it comes out of it's grooves-- it could get stuck in the open position if this happens.
I was confused at first too, but after I got the grip off I figured out what needed to be done by just examining the mechanics of how it works.