How to save the world:

1. Turn dangerous weapons(such as the key) that can end the world into the slayers sister.(By the monks.)

No one ever expects the sister. Just make sure another loved one isn't sick. Otherwise, a slayer, 2 witches, an ex-demon, a watcher and some random guy called Xander Harris might create a spell that ruins everything.

2. Get a gang together in high school and hope for the best.(By Buffy Summers.)

If you suddenly find out that your destiny is to save the world from the big bad, not to mention that your moving to a new school because someone burnt your old one (not my fault!) Then you should grab the first two people you see and hope that they either find out that their a witch or end up dating an ex-demon. If your lucky then maybe you'll end up with both.

3. Get a chip in your neck.(By Spike.)

First things first, you have to be a vampire or some sort of demon. Then you have to get captured by the initiative and get a chip that stops you from harming humans. Now, don't forget to escape. After that, the Scooby gang will probably let you stay, which will also give you protection from whatever's trying to end the world this time, until you find out that you can still fight demons and get yourself a crypt but still getting some protection. This is more to do with how to survive the end of the world than how to save it. Same thing.