Wash the first time with conventional powder, but put a little more than the norm. Thoroughly rinse jeans. But drying is not worth it, they need to still handle a few times. With the first wash will remove only part of the substance that was used in the manufacturing process of the product.
Now again wash the thing, but at a higher temperature and with the addition of 1 tbsp of baking soda in 5 liters of water. The washing machine takes a maximum of 10 liters per wash, number of water appliances you can see in the documents. Soda will not spoil the color, even the brightest fabric, so worry about the safety of the material is not worth it.
Rinse the powder the water should not foam during the penultimate rinse. And in the last rinse add a double dose of conditioning. In the next few washings, add soda and don't forget about the air conditioning. After 3-4 such procedures, the jeans will become softer cotton.
Pat has not yet dossie pants and do not put too high temperature of the iron. The heat also makes the denim stiff. In principle, they can not even ironed, if they are not strongly wrinkled.
If you want to buy soft jeans, visit a brand store. But on the normal market you can find soft jeans that you will like.