Summarize this article:

It will be much easier to imitate the howl if you know what it sounds like! Inhale slowly and consistently until your lungs are completely full of air. You'll need a large volume of air to imitate the volume and duration of an actual wolf howl. Cup your hands around your mouth as if you are going to shout. Begin with a low, mournful note, and then quickly increase your pitch by one octave: "A-woooooooooo!" Build up the volume of the howl so that it grows louder and louder. Hold the howl for as long as you can – at least a few seconds. As you feel your breath waning, gradually low your pitch and "fade out" of the howl. You can tell what a howl means by its pitch and strength. The longer the howl, the more powerful the emotion. Researchers also use pitch and volume to distinguish between individual wolves. Wolves rarely howl just once. If you want to imitate a wolf pack, then have several friends howl with you. Make sure that some of you howl in lower pitches, and some of you in higher pitches. Each wolf has a unique voice, and other wolves use these slight variations in pitch to identify who is howling.

Summary:
Listen to audio recordings of wolf howls if you can. Take a deep breath. Start howling slowly. Howl again.