years of practice........ i tend to drop the 2nd track in my headphones from beat 1 a few times and listen to the mix, adjust pitch accordingly, forget bpm's they give you a rough idea, but in my experiance nothing beats a good ear! to learn i suggest using 2 of the same track, get used to dropping the beat in time, this is how i discovered juggling hip hop may not be your thing, but i think all mixing uses these principles. hope this helps