1989 Chevy Beretta seatbelts

How do I replace my seatbelts I have a problem snapping it in because the covers have come off where you slide the buckel in and the driver side the shoulder strap has come off and I have to slide it back on or hook it on after snapping the main one in I put a ajuster snap thing on it it helps to keep it together but doesnt seem that safe where do I get new belts and is there some where I can get a manual that show how to do it or is it simple I hope I have a haynes but it shows nothing about replaceing the seatbelts thanks for any help