Now that you've added your Collections, you can add and remove projects from it.
First, click the Three-Line Icon to open the left side menu, and find your Collection. Next, click the Settings Icon for that Collection. In the Projects Tab, click into the Search Box to view all projects in this organization. Click all Projects you'd like to add to this Collection.
To remove Projects, simply click the X beside them. All changes are automatically saved.
NOTE: Removing a Project from a Collection doesn't delete the Project.
Find out more about Project Status.