So the recipe for a multi-site install (not using the subsites module) .
1. Copy the silverstripe files to a master silverstripe directory; ex. "silverstripe305"
2. Don't install it (perhaps thats ok too?)
3. Copy assets, mysite, & themes folders to another directory; ex. "mysymlinkedsite" [do I also copy the index.php?]

4. Set the correct rights to these folders [what rights are those 777 for write access of 655?]

5. Create symlinks to the shared folders (saphhire, cms, etc.)
[OK, this is old, my versions doesn't have a sapphire folder? and what about all the files that are just in the silverstripe305 folder not its subfolders? How do I symlink this stuff and which ones?]

6. Create a "silverstripe-cache" folder in your "mysymlinkedsite"
[just out of curiosity why? am I supposed to reference it in a file somewhere of will silverstripe do that automatically?]

7. Proceed with the installation as normal (by visiting e.g. localhost/mysymlinkedsite)
8. to install it on additional addon-domains in the future: copy assets, mysite, & themes folders to the newsites directory; ex. "mynewsymlinkedsite". and then do whhat? create another index.php for it? Symlink some files like in step 5 again...

Could you guys help me clarify, Thanks a lot. I really appreciate it.

Modify the _ss_environment.php

global $databaseConfig;
global $ASSETS_DIR;

if((strpos($_SERVER['SERVER_NAME'], '**') !== false))
{
$databaseConfig = array(
"type" => 'MySQLDatabase',
"server" => '',
"username" => '',
"password" => '',
"database" => '',
"path" => '',
);
define('ASSETS_DIR', '');

}

else{
$databaseConfig = array(
"type" => 'MySQLDatabase',
"server" => '',
"username" => '',
"password" => '',
"database" => '',
"path" => '',
);
define('ASSETS_DIR', '');
}

And change theme in the table siteconfig