You can add a note either using the Web Portal or the App.

On the Web Portal you can add them via the Contact sliding modal if you don't wish to leave the contacts list, go to CRM > Contacts > click on the edit pencil for the contact you wish to add a note to. An information panel will then open on the right side of your screen. Scroll to the bottom and there is a section for notes. Add the note and save.

You can then access the note when you go to the full Contact page.

You can add the note by going to the contact page via CRM > Contacts > Select Contact ID or Arrow > Select Notes in the Contacts Communication Section > + New Note.

In the App, Contacts > Select the contact > Select the menu button in the top right hand corner > New Note.

Note: All notes added in the communication section in the contact card cannot be deleted.