Navigate to the page you want add the custom search on. Right click on the search field
and then click on the 'create search' option.
The only thing you should have to fill in on the 'Search Engine' popup is the keyword field. You can choose a single character or a string of characters. The character(s) that you pick can be used in the address bar for quick searches. Lets say you pick the keyword linux. You can then go to the address bar and type: linux mythtv and Opera will search http://www.google.com/linux for mythtv
After filling in the keyword field you can select OK. You should now have a selectable 'Google Linux' search engine.
Another way to add search engines is via the Preferences Windows (Press CTRL + F12)
By the way, you can do this in Firefox as well. Firefox stores this information in the bookmarks.
