Blank shells are automatically created late in the previous semester and should be available to you (except in cases of recent schedule changes) well before the new term begins. However, they will not appear as "Current Courses" in Blackboard until the new semester begins. For example, and as shown in the following screen captures from FA20 and SP21, to access the next semester's shells, begin by logging into Blackboard and clicking the Courses link on the left menu.
On the Courses screen, you have two options for displaying your next semester's shells: (1) click the Current Course menu item and change that setting to Upcoming Courses, or (2) click the Upcoming Courses link, located toward the right side of the screen.
Once you can see your new shells, you can click the star icon on the far-right side from the course name to add that course to your Favorites list.