A wolf-like howl can be achieved by cupping the hands over the mouth and emitting a sustained and high-pitched sound. The sound must come from deep within and the head must be pointed skywards.
Take a deep breath so that the howl will last well over a minute. Begin with a slow and steady initial burst and then build up gradually. Never allow the howl to get too strong initially because it sounds more eerie if it gets louder over time. There are free recordings of wolves howling and the best way to learn is to practice imitating these recordings.