To add a piece to a collection:
Find the piece in your library or open it in the editor.
Press the 3 dots in the bottom right of the piece.
This will be in the top right of your screen if in the editor.
Hover over the "Add to Collection" option.
Choose the collection you want to add the piece to.
If you don't see the "Add to Collection" option, that's because you don't have any collections. To add a collection:
Press the "Create Collection" button in the left sidebar.
Give your collection a name, icon, and optional description.
Then create it!