Changing DOMAIN NAME of Sign-up page

Hi,

I'm going to change the domain name of my sign-up page only using the domain mapping plugin.

I don't need to change the domain site wide and will keep the original domain as the shared domain for user blogs.

I think I need to show Google and Co that I've moved to a new domain by way of a permanent redirect in .htaccess.

.htaccess files are like voodoo to me,

I'd appreciate some guidance with this.

What do I add to the .htaccess file and where in the file IF AT ALL.

Many thanks

Ralf