Create sub-folders within a deck

1. Select the deck you would like to turn into a subfolder
2. Drag it on top of another deck, a green check mark will appear
3. Drop the deck inside of the other deck. You have now added it as a subfolder to that deck

NOTE: If you share a deck that contains subfolders, you will be given the option to choose whether you share access to those subfolders as well or just the contents of that main deck alone.