Salt is a wonderful natural fabric softener. You can use coarse sea salt to make beautiful looking fabric softener crystals. Some people say that epsom salts make a wonderful fabric softener too that leaves their jeans feeling extra soft and smooth. Others say you should only use NaCl salts (like sea salt or Himalayan salt) because the minerals in the epsom salts can build up on clothes or interfere with laundry soaps. I haven't found that to be the case, but I'd like to warn you, just in case. It may be more prone to causing issues for people with hard water.
Add around 1/2 cup of the salt mixture to your laundry. You can either add it with your detergent (allowing you to add vinegar to the rinse cycle), or you can add it during the final rinse.