Rear Seatbelt Replacement Instructions

My local Toyota dealer quoted me over $400 for parts and labor in replacing my passenger side rear seat belt. I found replacement parts online for MUCH cheaper ($75 for both OEM rear seat belts).

Does anyone know how to replace it? A long time ago, a friend left my seat belt outside the door to where the buckle was dragging on the floor and bent it.

If not, does anyone have prior experience with the cost and service to get it installed? Your help is truly appreciated.

Thank you.