creating an unlisted admin Page
Using Jamroom
Take a browse through the templates that exist in your skin currently, you'll find lots of useful examples. What you're talking about here is the traditional way to do things. It has the most flexibility out of all the methods.
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If you're absolutely stuck for how to add the HTML code in that would be the same as what the editor would produce, make it in the editor, then click on the CODE VIEW button and copy the code from the editor to your template