As a non-believer, what is your moral foundation? What standard do you use to determine what is moral and what is immoral?
Treat other people as you would like to be treated
It is neither difficult nor divine
Oh I've borrowed from a number of things to build my personal code of morality. Thou shalt not kill was rather obvious. I borrowed from the Jedi (let's be real here, more the Tao) and eastern mythology. Ancient Roman and Greek mythology, and philosophy.
My family's hard work ethic rubbed off on me as well. People I've interacted with and looked up to, I try to adopt as many as their motivations and lessons as I can.
I try to observe everything, everyone, and every situation around me, and see it from as many angles as I can.
Then there is just common sense... don't trash the only planet you have, if you own a gun lock it away from kids, don't stick silverware in the electrical socket, don't drive under the influence. That I can just... recognize on my own, without outside help.
It makes sense to not do the things I don't want others to do.
Don't bring harm to anyone, physically or psychologically.