The input area for the mail itself is in the bottom half of the window on the Text tab. If you are not using a template, enter your text here. Maximise the view by clicking on Enlarge input field at the top right-hand corner of the Text tab to hide the recipient details or by clicking on the expand triangle in the top left-hand corner of the toolbar to collapse the entire toolbar including the formatting options.
Go to the Text tab.
Enter your mail text, copy text from other documents or edit the template as needed.
Format the text if necessary. In the toolbar, find the usual tools for formatting text and inserting style sheets, bookmarks, references, etc.