A quickstart package is a zip file of an already setup Joomla website, with extensions and templates installed by default and sample data.
First make a copy of the site's files you want to turn into a quickstart package, and remove the configuration.php file from this copy.
Download the package of the exact same Joomla version from the Joomla website, and copy its "installation" folder into your quickstart folder.
Open the database of the website you copied in PHPMyAdmin, click on the "Export" tab then select "Custom - display all possible options".
In the appearing options, check the options "IF NOT EXISTS" and "INSERT IGNORE":
Once done, click the export button and rename the file into "sample_data.sql". Open it and replace each occurence of the table prefix, for example "xxx_", into "#__".
Open the installation/sql/mysql/ folder of the quickstart folder you created and remove all its files except joomla.sql. Copy your sample_data.sql file into this folder.
Your quickstart package is ready, you can zip this folder and use it as a standard Joomla installation package.