It's best to take it to a pro. The pro will make sure that the springs are in good shape and replace any dodgy ones and properly tie them in. Ask around at local car meets to find a good pro.
Im a big fan of doin everything myself for free. I had 1 broken spring. I welded it back together and transferred it from the drivers side bottom to the center of the seat back where it will least likely be abused. Wasnt really that bad of a job to do on a set of saw horses with a piece of plywood. Seat feels perfect. No need to sub out everything especially something as easy as a old flat bench seat.
