Customize transfer confirmation e-mails template (FOA)

Question
How to customize the transfer confirmation e-mails to end users (FOA)?
Answer
NB: Please, note that, as a result of GDPR, many registries and registrars do no longer show e-mail addresses in the whois since the 25th of May 2018. For domains without (valid) e-mail address for registrant or administrative contact, the transfer confirmation e-mail (FOA) for an incoming transfer is not sent. The FOA for an outgoing transfer is always sent.
For a more visual explanation and deeper info about e-mail customization or "white labeling", refer to this blog post.
Any transfer of a generic top level domain (gTLD) needs to be approved by the domain owner or administrative contact via a link that he receives by e-mail. This e-mail is called "Form of Authorization", in short FOA. The FOA is pre-defined by ICANN, but allows for a bit of customization and personalization (white labeling). This is done via the reseller control panel, page Account > Settings > FOA branding. The following elements can be customized:
Subject of the e-mail. Can be written in your own language, make it less formal, or any other change. You can add the domain name in it by using the variable %%domain%%. An example might be:
Important: approve the transfer of %%domain%%
From address. Use your support department's e-mail address here, for example. Any valid e-mail address is allowed.
URL to terms and conditions. If you want to link to your own terms and conditions, you can provide the URL.
Confirmation link. In the e-mail, a link is included that the end user can click to confirm the transfer. 
Reseller name. If you do not want to use your own reseller name, provide here another name.
You can review the changes immediately in the example e-mails below the form. If everything is according to your wishes, press the Save button. Your customers will now receive the personalized white label e-mails.