I went through this just recently. I looked up a bunch of old posts. I had a few set of belts from different cars. One pair was damp and sitting in a zip lock bag for the last 6 years. They smelled up my whole garage.

I put the seat belts in a plastic bucket with a mild detergent I use for washing clothes (All detergent in this case). I heated a up a big pan of boiling water and dumped it in and stired it around. I did this 3-4 times. Each time I dumped it the water was black.

2nd thing I did was take each belt and scrubed them with a soft bristle brush. You need to be carfull around the white patch as it was easily rip off when its wet. After I scrubbed them I rinsed then in the sink and laid them out flate outside to dry. They looked pretty good at this point.

3rd thing I did was use the SEM vynile dye. This made the material very stiff and difficult to move through the adjuster. The wife did this part and I think she sprayed them very heavy.

Last I polished the metal buckles with the dremal buffing wheel and polishing compund. Mine had lots of rust and pits, but cleaned up pretty well.