In the Magic Bands section, go to "Manage Magic Bands or Cards". Find the existing Magic Band you want to use. Make sure the number from under the band matches the one on the screen, even if the design doesn't match. Then click "Activate". If it's already activated (in that case the only choices will be "Deactivate" or "Report Lost"), then you're all set.
It's true that you don't have to specially activate them for each trip, but if you have dozens of bands from countless trips over the years some might have gotten deactivated (happened to me), so you want to check and make sure the bands you all plan to use are activated.