Login to Joomla admin panel and then go to the articles edit section and in the desired article click Source at the WYSIWYG editor. The source code that you need to enter will look like this:
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<div align=”center”; margin-right: 10px; margin-bottom: 10px;”>
<object height=”258″ width=”321″><param value=”http://www.youtube.com/VIDEO PATH” name=”movie”><param value=”true” name=”allowFullScreen”><param value=”always” name=”allowscriptaccess”><embed height=”258″ width=”321″ allowfullscreen=”true” allowscriptaccess=”always” type=”application/x-shockwave-flash” src=”http://www.youtube.com/VIDEO PATH”></object>
</div>
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Note: You should replace http://www.youtube.com/VIDEO PATH with your video file URL. Note also that various options in the source can be altered according to your needs. You might want to align the video left or right, possibly alter the margin size, disallow fullscreen and adjust width and height according to the movie parameters.