You can either long tap on the date you’d like to add a note to, then select the hours tab or…

1. Tap on the day you’d like to add notes to
2. Tap on the Notes button in the toolbar (at the top)
3. Type your notes into the Notes box and tap on the back button (top-left) when you’re finished

Once you’ve added notes to a day, a small star icon shows against the day so you can easily see which days have notes.