The integration of the form needed for Google Co-op into WordPress is easy. More tricky is the integration of the results into WordPress. The obvious way, to create a page and to insert the code snippet, doesn’t work: no results are shown.
For my search engine I solved it by creating a template. The template is rather simple:
<?php /* Template Name: Your_Template_Name */ ?> Some HTML Google Search Result Snippet Some HTML
After creating this template, I created an empty page, and selected the new template as page template. And voilà, it worked.