To create a Folder, first navigate to your Library page using the left-hand sidebar. Click on the purple "Create" button located in the top left corner of the page, and select "Folder" from the dropdown menu. Give your folder a name and save.
There's more than one way to add Lists to your Folder:
Adding an Existing List
To add an existing list to your folder, locate your List on the Library page and click on the three dots on the bottom right corner of the tile.
Selecting "Move to Folder" from the menu will prompt you to select the folder that you would like the list moved to.
If your list is already in another folder, then clicking on the three dots on your List's tile will provide you with two options: move to a different folder or move to Library. Selecting the option to move the list to another folder will present you with the same window pictured above.
Creating a New List
To create a new list within the folder, simply click on the folder, then click on the "Create List" button in the top right corner of the page, where you will be prompted to enter a name for the list. Once a name is entered, you will be directed to the page of your newly created list.