Having played with this recently, here is a quick guide before I forget it. I had to use a little trick, I am not sure if there is an easier way to set it up. All right, let's get started. We want to use Google Custom Search since your blog deserves better search than the one that comes with WordPress. First go to Custom Search page and setup the new engine. Option worth noting is Advertising status where you can disable ads if you are non profit.