Adjusting the opacity in FCP

In order to generate a confused feeling for the audience and replicate the feeling of de-realization, I want to overlay many of my clips so that you can see a heart beat over a dog attack etc. This will be especially helpful when I have clips that have an emotional audio track but a very plain video. 


I found this tutorial: Adding Opacity to Files in Final Cut Pro

Read more: http://www.brighthub.com/multimedia/video/articles/72963.aspx#ixzz1VeZL8Zur

Much like the previous tutorial this was merely an explanation of what each of the tools did rather than how to use them, I personally find these a re better for my learning as I prefer to explore the program rather then going through a step by step process and duplicating stock standard effects.