Run shell script from a pkg file

I have developed an application and I want to create a package to distribute it. I am trying to create the package using packagemaker. But I got stuck in a problem. I want to run a script (postinstall script) from the package. Can anyone tell me how to write the script. In this script I want to copy some external files in some other folder. I do not want to include these files directly in .pkg (the package).