Multi site set up - please help very confused how to set up and configure

Could somebody please give me a step by step on how to set up a multi site drupal installation.

I have set up drupal for one of the site that I am hosting. I would use drupal for all the sites that I'm going to be hosting in future, but I don't want to have to copy or set up drupal in the /www/domain folder for every site that i host.

There must be a why that i can set up drupal once and host multiple site by pointing apache to a folder or a file.

Thanks