Follow the instructions on adding a search engine to Firefox by Mozilla.
Just by visiting the site, Chrome should have already stored it as a search engine. Just go to Chrome's search engine settings and find duckduckstart.com in that list, then click "Make Default" next to it.
If that didn't work try clicking here
to force it to be added. Alternatively try this link too here
DuckDuckStart should be easy to add to any browser or launcher (Alfred, GnomeDo, etc.) that supports custom searches.
Just use this search string in your browser/launcher's search engine options to create a custom search:
https://duckduckstart.com/?query=SEARCH(Replace SEARCH with whatever your browser/launcher expects the placeholder to be).
Please note: DuckDuckStart accepts both POST and GET requests. However, POST is strongly preferred, as it prevents leaking of search queries that are sent to StartPage.