P.M.
The simplest and cheapest softener is 1/2 – 3/4 cup of vinegar added to the final rinse. This not only has a similar action on the fibers as commercial softeners, which are acidic like vinegar, but it further cleans the fabrics, helping to strip remaining detergents out in the rinse. It's a good deodorizer, too. Towels and washcloths will come out more absorbent after a few washes if they have become coated with commercial softeners, which repel water.
If you need something that cuts static electricity in very dry weather, I add up to 1 capful of an unscented hair conditioner to the final rinse. Dilute in a cup of water first so it will diffuse through the wash, or you can get greasy looking spots.
I had to figure these out after becoming sensitive to all scented products. I also learned that most commercial softeners contain really toxic ingredients. No child's skin or lungs should be exposed to those nasties.