how to write a file in a writeable(777) website directory?

Hi friend!
Let's assume that i have a site which has 777 permissions on all its directory. People say its a vulnerability. Okay let me come to the point.
I have the following qns.

• How a hacker will use this vulnerability?
• How to write a file in that vulnerable directory?
• Or how to hack that vulnerable site?

Anyways this is purely for WHITE HAT. I'm testing my own web server with this vulnerability. So pls help me how to exploit this.