TinyLetter provides simple design tools, much like a standard email client. When you compose a message, you’ll use the options in the toolbar to change the styling of your text content, add images, add hyperlinks, etc.
If you’re looking for more customization, you can click the < > icon in the toolbar to view the source code and to edit your newsletter’s HTML.
You can also compose your message and send directly from your email client by following the steps in this linked guide.