Hi,
first enable the plugin (that do the redirection work) and set its option as your needs. If you are using sh404SEF maybe you need to activate "REDIRECT mode".
Now you are ready to use ReDJ, just add the items into ReDJ component. An item could match many URLs because ReDJ uses regular expressions. In this way you can find groups of Google 404 URLs that must be redirect to the same NEW destination adn can be grouped in the same item.
I know that this point is a little tricky, but believe me when I say once you learned how it works you will love it.
If you need help send me an email with a list of errors to redirect, and I'll help you to move first steps. You can find email address in "Contattaci" menu on the left.
Regards,
Luigi