To chant it the way Samael does can be helpful, but it is not necessary. Almost every mantra has numerous variations, depending on location, culture, tradition, etc. What is important is to chant the mantra consciously. Pay attention, and place your heart in your Being.

I recommend studying the lecture "The Mantra of Christ: Om Manipadme Hum." It is not about Bellilin specifically, but it provides a lot of very helpful information with regards to the practice of chanting mantras, which are applicable to any mantra we might use. The link is provided below.