Create your Module on your Jamroom install
Before packaging your module for the marketplace, first you need to develop your module into a working state on your own Jamroom install.
Once the module is working on your local server, then it can be exported from there into a package file by the Developer Tools
Its important to have a working module on your own system and package it from there.
This howto covers the point from having a working module ready to be released to everyone, to getting it available in the marketplace for one click installation.