Start at the Invoice Tab of the Invoice Module.
Find the Invoice to add a Credit to.
Click the Eye Icon to view the Invoice Details and Edit.
Scroll to the bottom and click Create Credit.
Enter the Price of the Credit to apply to the Invoice.
The Tax Rate will default to the Default Tax Rate for the Site the Invoice is on.
The default Description will say 'Credit relating to invoice ____'. If needed, enter a different description to identify the Credit.
Click Create Credit.
The Credit shows up as 'Credit [Invoice No.]' under Allocations and Credit Notes.
Click the Credit Link to view and edit the Credit.
A Credit is be removed the same way a Payment Allocation is removed from an Invoice.