Credit notes are always will be linked to an invoice that you have previoulsy created and you can add them either by going to the job's details or by going to the Invoice page.
Create an credit from a job's details :
1- Go to a job's details, go to the Invoice
2 - Click on the button "Generate Credit Note using this Invoice"
3- Inside the Credit note you can select a line and either delete items, change the values associated to an item (unit price, quantity, discount, total and tax) or you can add another item.
Please note that the credit note will be a copy of your invoice and should you modify the content keep in mind that the credit note's total cannot be above your invoice's total.
4- Finally, you can add a status to your credit note : Sent, Paid, Late or Canceled
You can also view the PDF, print it or send it by email by clicking on the "Credit Note" button.
2 - Click on an Invoice number
3- Click on "Generate Credit Note" in the drop-down list
4- To the message "Add Credit Note?" click on 'Yes' to the message "Add a Credit Note?"
5 - Go to "Credit Notes" click on your your credit note's number and click on "Edit"
6- Edit the content of your note (as seen in the previous paragraph) and click on "Save"