How to sound stern without sounding mean?

People have told me that when I tell someone to do something I sound meak. And then if I try NOT to sound meak people say I sound scary or mean when they've talked to me way worse. And even when I do talk "scary" they still won't do what I tell them to do. And I'm usually not bossy, infact, most people try to boss me most of the time. So how can I seem stern without seeming mean or scary?