How do I repair/replace the seat belts in a '72 super beetle?
Repair? Why? If the belt straps are deteriorated you need to find a restoration shop that will re-web them.
Or you need to find a company that makes reproduction seat belt assemblies.
Or you can buy New-Old-Stock original belts if you can find them.
Google is a better friend than CarTalk for your search.
You can buy replacement 3 point seat belts for $83 each, unbolt the old belts and install the new belts.