One way to soften stiff leather is to warm the item outside in the sun for 10 minutes and then massage it with coconut oil. As the leather absorbs the coconut oil, it gets softer.
Another method to soften stiff leather is to wipe down the item with a wet cloth and then let it dry completely. Next, rub a leather conditioner into the material, and let it soak for 30 minutes. Repeat this process until the leather is the desired texture. Different leather items may require different conditioners. Leather boots are typically softened with a lanolin-based conditioner, and lighter leather items soften well with a rawhide cream.