Hello,
the easiest way is to start the regular scenario, put the vessel to seelp-mode (or cockpit off-mode) with using the
check list, and quicksave it.
Then you can simply drag-n-drop the quicksave to the folder of the scenario (or maybe your own folder) and rename it.
I have already modified many scenarios by using this method. For example you can copy'n'paste parts of other
scenario-files (maybe vessels, stations or orbiting data) into your desired scenario, and run it first to make all further
changes, before you really play it.