Before going into the implementation let me first share the screen shot how the popup will look like (and obviously you can customize it as per your branding needs)
Just download the css, images and Java script files from below link
Above download will give you a demo html page, you can just take a look and copy pastes the code on your page. This works great!!
For more detail on the JQuery modal dialog box you can visit to http://www.ericmmartin.com/projects/simplemodal-demos/