To add a piece to a Collection:
Find the piece you want to add.
Press the 3 dots in the top right of the piece.
Hover over the "Add to Collection" option. A pop out will appear with your Collections.
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!