You'll have to create an event that's called "Set Key (float/string/etc", the Key input is basically the name of your save (not the entire thing, but only one variable), and select whatever variable you want in the second input.
After that, you'll have to create a "Get Key (etc)" in the beginning of your game, to reapply those saved variables.
It doesn't cover multiple save files, merely simple saving and loading.
