I can walk you through it but firstly I have to say that there are many different ways to install a web client and this is the one that works best for me.
I always install squirrelmail as a backup and run squirrelmail-configure
Then I even do the symlink to /var/www/webmail add a mailbox and then log in to squirrelmail and send receive some mails. This ensures that my MTA is working properly and no other issues will get in the way of my roundcube install.
Since you have already installed roundcube you should do this:
apt-get remove --purge roundcube*
You will be asked whether you want to deconfigure the database so say yes here.
While you are removing roundcube you should get a list of packages that have been installed as roundcube "depends" ...
So, do not run apt-get autoremove
Instead, make a list of those automatically installed packages and manually install them with apt-get install <package names>
Log in to phpmyadmin and make sure that the database has been removed completely.
When you are satisfied that you have returned your system to its normal working state before you installed roundcube do this:
rm -rf /var/www/webmail
This only removes the sym link .. you still have squirrelmail in /usr/share
Follow the instructions from here:
To create the proper configurations and database.
Before making a symlink to your mail location run this command:
mv /usr/share/roundcubemail-0.3.1 /usr/share/roundcube
This simply moves your entire roundcubemail-0.3.1 directory to a new directory called roundcube in /usr/share <--- replace the version number with the one you are using.
In my case since I am using Debian, I make a symlink to /var/www/webmail like this:
ln -s /usr/share/roundcube/ /var/www/webmail
In your situation - as you have pointed out - your symbolic link from /usr/share/roundcube to your webmail location will be different.
You may have a few log in issues to begin with but these can be easily fixed.
Tell us how you go.