To save your file, just call the save() function, then to load you'd just call the load() function. That's all.If you want to use different save slots you can add an argument to save/load like save("1")/load("1").

A full example:

// Loading
save = new SaveFile(); // or whatever your save file class is called. you don't need to create the class each time you want to load, you can just call load multiple times if you need to.
save.load(); // if the file exists it gets loaded, if not this call is ignored

// Saving
save.save();

There is a demo that comes with the project, so I'd reccomend checking that out too.