To add private todo items (or check-off any existing private todo items), make sure the My Todos tab is selected, then select either the Private Todos (This Page) or Private Todos (All Pages) dropdown filter option,

After either private todos filter is selected, you can give your new Private Todo List a name, then click on the Add New List button (or press the Enter key) to create a brand new private todo list to start adding items to.

Once you've created one or more private todo lists, you can start adding private todo items to them by adding the todo item text in the input field under the todo list of your choice, then click on the Add Todo (or press the Enter key) to create a new todo item in that private todo list.