Create invoice with custom fields from order (newby)

On my salesorded and invoice form I have added some custom fields for billing references.

When I press the Create Invoice button from the sales order, I need the fields to be copied on the invoice.

I found this wonderful post and learned how to update my salesorded then call the process order function.

But I cannot figure out how to update the newly created invoice.

Call invoice processorder

As I as sure it is obvious, I am just getting started so this is all very new, but very exciting.

Thanks in advance for any help you can give me.