How did Abel know to sacrifice animals to please God

Well, Way back in the time of Adam and Eve, Cain and Abel God was more personal he could really closely communicate then. He can still do so now but He doesn't. God Requires sacrafices to make attonement for someone's sin. Abel sacraficed the first of his flock (probably the best) to God. Because Abel gave his best to God, God was pleased. Cain however, didn't give God his the first and best of his fruit. That's why God was displeased. (I think, I don't know the mind of God)