Click on a colour and then select add note. Told you it was easy.
A new note is added to the top left of your notepad. You can write on this straight away by clicking in the middle of it. You can drag it around by clicking and holding the mouse down around the edge or you can resize it using the bottom-right corner.
Right-click on a note to remove it or change it's colour
Once you're done updating your notes, hit the save notepad menu option. Squareleaf does not auto-save.
If you've made some change to your notes that you are not happy with then just reload your previously saved work. The Logout and Home options kinda do what you'd expect.