3021 redirect when domain is mapped

I have a multisite install and have created a site for a client where I have mapped a new domain to the site.

Everything is working on the new site as it should be. Now I want to take the clients old domain and map it to the new site.

As the new site has some different file names I would like to make 301 redirects for google etc. Can anyone tell me the simplest way to do this? If it's in an htaccess file where would I create and host it?

Thanks