[mbreese]

Somewhat ironically, you mentioned the Google results were unhelpful. But, the DDG results for this search look pretty reasonable.

https://duckduckgo.com/?q=add+duck+duck+go+to+default+search...

The first hit: https://duckduckgo.com/set-as-default

It even has instructions for how to add DDG if it isn’t in the default list.