A scrolling list of your aliases will appear and you can click on the person you wnat to send to and then click on one of the header buttons. The best way to learn is by doing, so go ahead and send yourself a message. The cursor should now be in the subject field. These people will receive the message but no one else will know they did. This practice may strike some people as tricky, but it's an accepted practice in business.
Many people decide what to read and what to delete based on the subject line of a message.
If it is, you're ready to start typing your message. When it's time to start a new line, Z-Mail starts it for you. It's a good idea to break your text into paragraph-sized chunks because very long passages of text are hard to read on the screen. Under the Edit pulldown menu there are commands you can use for editing your message. Like a simple word processor, Z-Mail allows you to select text to cut, copy and paste.