WordPress.org

1. Upload 000-multiply.php to your plugins folder, usually wp-content/plugins/.
2. Activate the plugin on the plugin screen.
3. Head over to the "Presses" submenu under "Manage" to create a new blog.
4. Use the dropdown in the top right-hand corner of the screen to change to the new blog.
5. Set up the blog options. It works best if each one has a separate 'Blog Address (URI)'. (Options->General)
6. If you use mod_rewrite, make sure to create a new .htaccess for each blog.
7. Create a new index.php file in the selected Blog Address directory. (See below.)
8. Enable pingback. (See below.)

X. Put upgrade-multiply.php in wp-admin/. Your alternate blogs will need to be upgraded separately when you upgrade your main blog to the next version of WordPress, and that's what this script is for. Until it actually comes out, though, I can't be 100% sure that the script is going to work properly -- caveat emptor.