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In order to create a custom server assembly, you must already have a Geronimo Server installed and started. Double clicking the Geronimo server will bring up the server editor in the main window. Items for working with the server can be found on the Plugin and plugins group page. Clicking on the Plugin tab and selecting Create Custom Assembly will bring up the New Server Custom Assembly dialog that is shown below.
Enter in the pertinent information and select all the plugins that you desire to have in the new server. Press Ctrl key to select multiple plugins. Geronimo Assembly :: Boilerplate or Geronimo Plugin Group :: Framework MUST be selected in order to create a working server. The working server will be placed in the specified directory. A tar ball or zip file will also be placed in the var/temp directory of the running server for easy distribution.
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Now that we have created the custom server assembly above, it is time to use it. Stop the original server and uninstall it (easiest done by selecting Windows->Preferences->Server, and removing the server runtime environment. See Defining Geronimo server runtimes and servers page for the detail). Use the same method that you used to install the original server with one small difference. This time, point to the directory where the newly created server is.
In place of the full Geronimo server that you started with, you should now have a valid custom Geronimo Server.