Glad you asked...
There is really no one way to ask for your old job back. The best way would be to simply go in their and ask for it back in a sincere fashion.
Then again, this answer might help you...
If you leave a job, you never want to burn any bridges because you never know if you will ask for your old job back. Try to stay connected with people that work there. If you were a really good worker, then you may get lucky and the owner will call you and ask for you to come back.

