How can one conceive twins naturally?

This might sound stupid I know, but I've seen alot of women on here say they're carrying twins. I want to know if there's a way, besides going to the Dr and putting multiple babies in you, to have twins? I have tried looking online and no luck. If anyone could give me some good advice that would be great, and no I'm not a teen or anything. I'm a 26 year old mother who wants twins. Thank you!