Sample of an email stylesheet for invoice

When we reject an invoice, we want to send an email to the service provider. I want to create a custom version of the email and stylesheet is like Chinese for me, I'm very green. 😊

I don’t know if someone have something I can start with? An example that would allow me to understand where I should get my information. I tried to start with an existing stylesheet but I can't get the invoice information on the email ☹

Thanks in advance

Martin