How to link featured images to external URL

Hi,
I’m using the free “Simple Grid” theme http://www.dessign.net/simple-grid-theme-responsive/ and I would like my featured images from the front page to direct the user to an external link (amazon link). Using custom fields seems a good idea. I am not familiar with coding and the solutions I’ve seen in similar threads seem to be “custom made”. As I don’t know how to adapt the code sequence to my theme, I was wondering if I can get any help…