A basic rule of thumb for washing towels is not to mix them with clothes, and to avoid high temperatures when washing and drying in the washing machine, because the heat can damage the fabric fibers and make the towels rough.
The second step is to use fabric softener with the addition of a small amount of vinegar and lemon juice, as the acidity contained in vinegar and lemon helps to remove limescale and soften the fabric fibers.
Sodium bicarbonate can also be used because it is good for preserving fibers and for better cleaning of towels. All you have to do is to add 100g of bicarbonate to your regular laundry detergent for a great clean.
It is best to reduce the use of soaps and detergents, as soap residues can build up in fabric fibers and make towels rough. To keep it soft, use less detergent so your washer can remove the soap residue.