In the create page on DashingDon, next to the option to activate the save plugin, there’s a link How do I use this? with very simple instructions, which I reproduced below.
Place the following code snippet in your startup.txt file before any story text:
*sm_init mygame | 3
Where ‘mygame’ is a unique identifier for your game (to prevent other games from overwriting saves) and ‘3’ is the number of save slots you wish to allow for your game (more than 6 is not recommended).
More information on this plugin available here.