Click "Items" tab (1), expand "Data Source Fields" list (2),  drag selected field (3) to the canvas.

More details about "Data source fields" see in "Working with Source Object fields"

Virtual fields are inputs that allow user to provide various type of values without saving them to the base. Virtual are used to provide more complex scenarios of interaction between user and a process.

Click "Items" tab (4), expand "Virtual Fields" list (5),  drag selected field (6) to the canvas.

Options section contains other different elements and components:

• form section
• nested form (allows include one form into another)
• custom button
• lightning component
• custom list
• blank spaces
• static text (plain text or HTML)
• custom links
• file input
• image input and preview.

Click "Items" tab (7), expand "Options" list (8),  drag selected field (9) to the canvas.

Click "Items" tab (10), expand "Related Lists" list (11),  drag selected field (12) to the canvas.

Select component (13) that you want to remove and click "Delete" button (14).

Select form elements using "Shift" key and click "Delete" button (14).