what are the steps to install magento module/extension, what is the state of compiler? on or off and is compiler should be on after module/ extension installation ?
Generally, you would :
- Turn off compilation
- Back up files (optional)
- Back up database if it will be affected (optional)
- Turn on the cache
- Upload extension files per extension developer's instructions.
- Refresh the cache
- Log out, then log in.
- Re-enable compilation if you are using it.
That should take care of the installation.
There is a free, comprehensive guide here that has more information : http://store.fooman.co.nz/ultimate-magento-extension-guide