Simple Cross-Domain Iframe Script

I need to be able to show an HTML box over websites I do not own. I am told this can be done easily using iframes but I do not know how to do it. A perfect example of this can be seen here: [login to view URL] You will notice the box shows over the top of the website and the website is dimmed in the background. If you click on the button "Skip" then you can see how it directs you to the intended website and bypasses the form. I want to be able to do several things with this HTML box so I need to be able to resize it and show no scroll bars vertically or horizontally. I have my own website to host the scripts so installtion is not an issue. Thank you.