- download and enable simplenews and mimemail
- admin/settings/simplenews/newsletter: set format to HTML and save
- go to node/add/simplenews, and make sure Wysiwyg is disabled (click on Deactivate rich text, for example)
- paste full HTML in the body section, and send your email. (Actually creating good HTML is outside the scope of this tutorial!)
- you might have to run cron on your site for the mail to be sent.
You might also want to look at https://github.com/alberto56/generatecivihtml to generate the HTML code.