Credit Notes in Karmly are for applying one-off lump sum credits to your clients. This would generally only apply to permanent fees being credited, as any timesheet issues should be corrected in order to keep proper track of margin in your account - see Time Credits for information on crediting time.
To add a credit note you'll need to first add a Placement with the Placement Type set to 'Credit Note'.
When you're ready to charge the credit note, you can simply Create an Invoice Batch normally, using the standard Invoice Template that you use for permanent placements.
Once the draft invoice is created, modify the text in the invoice header to note that it's a credit rather than an invoice.
Alternatively, if you find you need to issue credit notes often you may prefer to create a separate invoice template. That might look something like this:
Adding a Subtotal Group filtered to 'Credit Note' (as pictured) will ensure that using this template for an Invoice Batch will only pull in Credit Note type placements.