The Skeleton of a Drupal 7 multisite guide can be as follows :
Install Drupal 7 the normal way on the first domain.
Make your DNS work for the second site’s domain (2ndsite.com) to point to same IP.
Make your Web server work for the second site’s domain to point to same directory. (for apache this can be adding the second domain as an alias in the current host or making a new virtual host that has the same document root)
Make a new directory with name equal to the second site’s domain in under the sites directory (drupal7/sites/2ndsite.com)
5.a. chmod the second site’s directory (drupal7/sites/2ndsite.com) to 777 (change permissions so that they can be modified by anyone)
5.b. Make a new directory called files (drupal7/sites/2ndsite.com/files) and chmod this new directory to 777.
Copy default.settings.php to the new site’s directory as settings.php (drupal7/sites/2ndsite.com/settings.php) and then chmod it to 666
Navigate to the second domain (2ndsite.com) and install Drupal normally.
more information please refer to: drupal wiki.