To mark yourself safe on Facebook during a crisis or emergency, you need to use Facebook's Safety Check feature. Here are the steps:

1. Open the Facebook app on your phone or go to Facebook on your computer.
2. In the search bar, type "Crisis Response" and open the page.
3. Scroll through the recent crises listed and select the relevant one.
4. On the crisis page, tap or click "I'm Safe" to mark yourself safe.
5. This will send a notification to your Facebook friends and share a post on your timeline indicating you are safe.

If you receive a Facebook notification asking if you are safe during a disaster, you can simply click "I'm Safe" to mark yourself safe quickly. You can also ask if friends are safe by going to a crisis page, clicking "Safety Check," and then asking friends who haven't marked themselves safe yet. This feature helps keep your friends and family informed during emergencies.