Google offers two versions of Custome Search Engine, one is simple HTML-based, and the other one is Ajax-powerd that display inline search results. Now, you don’t need to register, or sign up for anything, with just three clicks, you can add a Ajax-powered CSE to your site. All you’ve to do is – just visit this page, configure your CSE using any of the following three options: “Automatically search my site, Search my site and use AdSense for Search, Use an existing custom search engine”. Enter your AdSense or Custom Search ID, if you select 2nd or 3rd option, preview your CSE, then copy CSE code, and paste on your sites HTML.
Source:→ Matt Cutts