Applying a credit to a sales Invoice

I have read all all the help manuals but still not understanding the following :-(

1. How do I apply a credit note to a particular sales invoice in QB which is yet to be paid. I have:

1. turned off Automation - Automatically apply credits
2. Entered ab adjustment note  which is reflected as unapplied in the status column
3. But not sure how to apply it??

2. How do I apply a credit note to a  particular closed sales invoice where the customer is seeking a refund to his bank account.

3. How do I apply a credit note to a particular closed invoice where the customer would like the refund to be applied against any future invoices.

Thank you