Canvas notes can help keep a flow readable to the people editing it.
Navigate to the assistant builder by clicking the hamburger menu in the top left and selecting "Assistant builder".
Create a flow or open an existing one.
Double-click the canvas background where you would like to add a note.
Select the node type "Note".
The note is added to the canvas and selected for editing.
Enter the text of the note. Any URLs are automatically turned into clickable links.
As with other steps, you can drag it around the canvas to relocate it or click it to edit it.