Magic Bands are already "active" and all you do is assign them to someone through
MDE and attach a park or other reservation to them. That's why you can use a MB to "scan" in to your hotel before you even step foot anywhere near a park. If you buy the MB through Disney prior to a hotel stay, in the order process you assign the band to a guest and they arrive from Disney already set up (assigned to that guest). If you purchase a band through
ShopDisney, or in a gift shop ort other merchandise location, there is a pretty simple process included in the package for assigning the bands through MDE. All of that can be done long before your trip.
If you are planning on assigning an annual pass or theme park ticket to a Magic Band, or adding a resort stay or ADR, those can also be done before hand as the actual ticket or pass ticket doesn't activate until the band is scanned at the park.