Okay, I've been and done my homework even if you girlies haven't.

1. You download the stumbleupon toolbar for your browser type.
2. When you find a site, video or image that you like you click on the blue '

' image near the left end of the toolbar. It will turn green, so you know you've done it right.
3. If that exact url hasn't been 'stumbled' previously, a little window will open up and ask you for some details about the site. Not much, just title, description, category & whether it's adult content.
4. You're done! You've stumbled a site.
Now for part two. You can also 'go stumbling' by clicking on the little green and blue icon or the word "Stumble!" at the very left end of the toolbar.
This will take you to a random site that someone else has already stumbled.
At some stage, you will be asked to tick some boxes to mark your interests, so that StumbleUpon can show you more relevant sites.
As time goes on, it's supposed to get more accurate, depending upon what you give the "

" or the dreaded "thumbs down" to.
Does that help you out at all?
I've gone stumbling for about one and a half hours over the last week and I've noticed that I'm beginning to get traffic from stumbleupon now.
Not a lot, 10 yesterday was the most so far, but it's a start...