how to add a search engine

I've recently acquired an iPad Air 2 & have downloaded opera mini to use as a browser. I'd really appreciate some simple and clear advice on how to add search engines (specifically start page, and duck duck go) to the browser so I can have alternatives to google. I use these browsers on my desktop & am happy prefacing searches with d or sp as required. Have failed so far to find any help so am hoping a forumite will help...many thanks.