Sticky Menu same as Joomlart website

Hi I love the way the menu works on the Joomlart.com website. I would like to incorporate the same effect on my website. I would like the user to be able to scroll slightly until the menu touches the top of the browser and then for the menu to be fixed as the user continues to scroll. I have looked online and through the discussions, but I can only find a way to have the menu fixed to the top at all times. I would like the user to be able to scroll first and then have the menu become fixed.

Can someone please let me know how I would achieve this? Many thanks

Stu