Here i show how to install joomla 3 quick start package
package which you downloaded from ThemeForest.net then go to Quick Start Package Folder >extract the xxxxx-quickstart-packagej31.zip then Upload all files to your server.
(for example: http://www.your-site-name.com/ or if you use any folder then must be type http://www.your-site-name.com/folder-name/) so you will get this snapshot like installation page, just fill then and click next:
and fill the database information here and click next:
This is the important step for get demo like looking, you need to check the Sample Data radio button to get demo like look, and finally click on install button.
So your installation process will be run like that:
after installation done you will get like this page and you need to click remove installation folder button:
Ok you are done now your site ready to use: Have fun :)