To mark yourself safe on Facebook during a crisis or emergency, follow these steps:

1. Open the Facebook app or website.
2. Use the search bar at the top to search for "Crisis Response" and open that page.
3. On the Crisis Response page, find the current crisis affecting your area.
4. Click or tap on the option "I'm safe" or "Mark yourself safe."
5. This will notify your Facebook friends and post on your timeline that you are safe.

If you receive a notification from Facebook about a crisis in your area, you can simply click "I'm Safe" in that notification. You can also manually mark yourself safe by going to the Crisis Response page even if you don't get a notification. This feature is useful in natural disasters, emergencies, or other crises to quickly let loved ones know your status.