You don't, at least not easily. Don't install anything on the hard drive in
question.
Using another computer, search the internet for programs and/or business
that can recover a hard drive from a format command. A Google search using
"unformat hard drive" revealed many supposedly free programs that will
unformat the drive.
Basically you are SOL. If those files were important you should have backed
them up before reformatting. You can try to find something that can extract
your files but *I* wouldn't put any hope in that.