First, search for the contact you want to to add a note to by using the Quick Search or Advanced Search.
Your search will return the contact(s) that matched your search criteria to the contacts form.
On the contacts main screen, we have 12 CUSTOM FIELDS in which you can place any piece of data of any size. Simply double-click in the custom field and a full screen window will pop up with the custom fields data allowing you to add/edit the data.
The Additional Contact Info tab also contains 12 more custom fields in which you can add any amount or type of data that you want to any record.