1. While logged into your account, hover over your user icon and select “My Works”.
2. Click the “Edit Works” button.
3. Tick all the works that you want to transfer into a given collection.
4. Click the “Edit” button.
5. In the Associations section of this page is the option to “Add To Collections”.
Type in the start of the collection name and it should appear in the autofill. (Don’t change anything else, or it will overwrite what was originally on the work.)
6. Click “Update All Works” at the bottom.