For installshield you have to first record the installation that you want with the /r switch.
For my example I'll a monitoring agent I use on my servers called eGagent. It has a setup.exe. I open a command line window and navigate to the directory with the agent and use the following to start the process of recording the iss file.
example:
setup.exe -r -f1"c:\eG\eGsetup.iss"
This will launch the setup application and you select the options you want and they get recorded to, in my case, c:\eG\eGsetup.iss and record a log file in case I messed up.
Then if you want to silently install the application you use
example:
setup.exe -s -f1"c:\eG\eGsetup.iss"
That's it.
Hope it works out.