How to do an overlay with scroll

How to do an overlay like on this site: http://marcosilva.co.uk/ under “Work Section” when you click a project an overlay pops up with nice scroll. I tried to do it, but whenever I add an overlay I end up having two scrolls, one for the background and one for the overlay, and then when I scroll the page, the background also scroll down, how to get only one scroll and the back page would be fix like the one on link above? Thanks

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