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The basic structure of print.php is to create a function that pulls variables from the article, either by direct reference or by including the class used to generate the article content.
Unfortunately, there are no hard and fast rules about how to achieve this, so I can only suggest you download a few modules and compare their print.php structure until you see one that suits the module you are working on.
It's worth doing, especially if your users will be using Firefox which hasn't got a remote chance of ever printing a web page properly.
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