Check out this video for a full walkthrough!
Action items (or todos) are represented by the square (☐) in Fellow notes. There are 4 main methods to add action items to notes. These 4 methods can be used in 1-on-1, meetings, and private/shared streams.
Here are all four in action:
To break those down ....
Method 1 - (+) Icon
By hovering your cursor over the far left of the note (the edge) you will see a (+) sign. When you click on this (+) sign, you will open up this menu where you can select the "Action Item".
Method 2 - Keyboard shortcut 
If you are all about speed and efficiency, you can use this keyboard shortcut (it's super handy)
Method 3 - Toolbar
First type out the text that you want to be an action item.
Then highlight that text, which will open the toolbar.
Then select the square icon to make that text an action item.
Method 4 - Slash Menu
You can also open up this menu by using the slash ( / ) command. Then select "Action Item"
You can create an action item directly in the action item section by clicking on the plus button at the side of the note. (Something like this:)