To mark a region for cutting and pasting you press the left button and drag over the region. When you release the left button the mark disappears but the region has been stored in the emacs memory. The mail summary buffer which shows you all messages. The mail buffer which shows the specific message which is highlighted in the summary buffer. Some new entries appear in the menu bar which allow you to manage the mail, reply, save, delete, forward, etc.
Subject is optional.
Then type in your message as if you were typing an emacs file. You can apply all editing commands applied to a regular file.