Other than creating the jail, which is thoroughly described in the manual, nothing about your question has anything to do with FreeNAS. A jail is a FreeBSD installation. Search the web for guides on installing this software under FreeBSD. It's highly unlikely that there are any guides for installing it under FreeNAS, and there's no reason for there to be such guides.
If you're having trouble with a specific step in the process, let us know. But "I need clear guidance" says that you haven't even tried to research the question.