I needed to redirect 404 not found errors to the default home page in Umbraco.  Usually in Asp.Net you can just set the web.config and set an error statuscode to do something.  However in Umbraco it uses its own internal 404 page which has a neater way of redirecting to a page you specify.

First login to your Umbraco site.  Go the page you want the 404 to be redirected to, here I am using my Home Page (default page).

Notice the URL and the last 4 digits (1012).  Remember this bit.

Open the UmbracoSettings.config - this is usually in the Umbraco Config directory.  Find the <errors> section and specifically <error404> e.g.

Then enter the URL ID as seen above and now any 404's will be redirected to your home page (but the url will still show as the incorrect path).