Multi site setup - help needed

Can you see where I am going wrong in trying to set up Drupal for multisites?

I have 3 domains on the same server - www.fruit.com is the primary domain. www.apples.com is an addon domain
and www.oranges.com is an addon domain

In my non-drupal stage I have these folders:
public_html
public_html/apples
public_html/oranges

At the moment when I type:
www.fruit.com,I see the pages in public_html.
www.apples.com correctly goes to the pages in public_html/apples.
www.oranges.com correctly goes to the pages in public_html/oranges.

So far so good!

Now I want to make this a Drupal site. As these sites are not presently active I am doing everything online.

I installed Drupal into public_html/drupal
So now I have:

public_html
public_html/apples
public_html/oranges
public_html/drupal

I ran install.php (www.fruit.com/drupal/install.php) and set up the database sqlapples

Now www.fruit.com/drupal takes me to www.apples.com which is what I want and all works fine
I have the first site up and running.

Now to turn this into a multisite installation sharing common code with www.oranges.com but using
separate databases for each domain (www.apples.com and www.oranges.com)

I set up a new mysql databases- sqloranges.

In /public_html/drupal/sites I created a folder called apples and a folder called oranges

Into each I put settings.php.

In the sites/default/settings.php the $db_url contains the apples database data and the $base_url is remmed out

In the sites/oranges/settings.php the $db_url contains the oranges database data but $base_url = ?????
(index.php is in /www.fruit.com/drupal/index.php). Do I leave the $base_url remmed out or should it be :
$base_url = 'http://www.fruit.com/drupal'; or what????? (have tried numerous variations but no luck)

Then I renamed my existing folders, so I have
public_html
public_html/oldapples
public_html/oldoranges
public_html/drupal

Then I created this makesymlinks.php which I put in public_html:

symlink(".","apples");
symlink(".","oranges");
print "Links Done";

I ran this (www.fruits.com/makesymlinks.php) and it created these links in public_html:
apples
oranges

So now I think I am ready: I go to my browser and type: www.oranges.com
Instead of starting drupal as I was hoping, it shows me the files in the public_html folder.

I've tried following threads on this topic (particularly node/117658 & node/107347) but am new to this and don't know what to do.
Please can you see what I have done wrong?

Carol