I am wondering, if I want to use an older firmware on the iPhone, how can I specify that?

I know that with alt+click on restore i could locate the file. I also happend to know that I can download any version of the firmware from our great site, ModMyI.com .

But what I don't know, that what should I do with the unzipped folder, that is downloaded. What should I select, etc.

Thanks for the help,