The HTML email is written just like a web page. The code must appear in the Source section of the email. Not knowing which email prog. you are using, it would be hard to tell you where to place the source. Outlook doesn't even require a source; just make sure that the new message is sent as html. The content-type in the message header must be changed from =plain/text to =text/html.
Having said that, and again depending on your email programme, the HTML email sometimes sends both versions in the same email; one as HTML and the other with all the codes.
If you are trying to create an elaborate HTML email with pictures, tables, etc., the message may become garbled because not many mail programmes understand complicated HTML tags.
Hope that helps. If you want to tell us which email prog. you are using, I'm sure there will be a solution to your problem.