Ginger,
Cockatoos are tricky because they are so light, especially the little ones. If you really want to slow Truely down, make sure not to get the "show clip", ie, don't let them convince you that she'll be prettier if you leave the first flight or the first and second flight and just cut two or three of the next ones. That will not stop her one iota, especially if she's a good flier. If you've never clipped a feather before and you're going to go to the vet, have them show you how to do it because then you can do it on your own later on. It's very easy to do, especially on a white bird.
If you look at Truely's flights, you might notice that she has some blood feathers growing in. As long as you don't cut at the blood line, you'll be fine. Generally, people who trim wings will clip a few inches off of each flight and you can start gradually with the first or first and second flights on each wing and see if that will get the desired "stop" you are looking for. If not, you can go one more flight.
There are all sorts of diagrams on the internet, but it's fairly easy to do and yes, the closer to the body you clip, the more likely it will effect the flying ability. Obviously, you don't want to cut the feather right at the base...but I'm assuming you already figured that one out!