Place the .htaccess file in the root directory of the one you want to redirect from. For instance, if your main site is a dot com and you want your dot net to forward to your dot com, add the following code to your .htaccess file:

Options +FollowSymLinks
RewriteEngine on

RewriteCond %{HTTP_HOST} ^www.mydotnetdomain.net$ [NC]
RewriteRule ^(.*)$ http://www.mydotcomdomain.com/$1 [R=301,L]