Go to the directory where you uploaded the script.
In your command line issue the following commands:
chmod 777 backup
chmod 777 tmp
chmod 777 includes/config.inc.php
chmod 777 language/<language>.php
chmod 777 templates/<template_name>/css/style.css
Where <language> is the actual language you use for the script, and <template_name> is the actual name of template.
In our example we will use mysite.com for fake domain name and dir/ for script directory.
You should replace it with your actual domain name and directory where you installed the script.
In your browser type in http://www.mysite.com/dir/install/index.php (remember to put your real domain name
This will run installation script.
Fill in the form (for complete description of fields refer to online manual)
and click START INSTALLATION button.
Configure script from your admin panel.
IMPORTANT! After installation is done REMOVE 'install/' subdirectory in your script directory.