This is not 100% correct ( I follow the Chinese folk religion ). It tends to rely upon two things:-

1. Examplary life ( ie:- you really need to as a human being lead a moral life OR if not a moral life then have done a single deed so heroic or so good or so amazing that it is worthy of praise. Most of the time this single deed causes one’s death or if not directly cause it leads to one’s death years later )
2. Post death, a miracle must have had happened. People must either have witnessed the person ascending to the Heavens, or blessings or healing associated with the person occurs.

Once this two things are met, and the person achieves a strong but informal following, then Canonisation happens when one is acknowledged to be a God.

This is how Mazu became a Goddess, how Guan Zhe Zhun Wang became a God etc.. how Guan Gong became a God. Once upon a time they were all normal human beings like you and me. In the case of Mazu as Lin Moniang She was such a helpful, kind, heroic and brave teenager and then later younger adult then when she finally died people mourned her, only to have Her come back as a Goddess guiding their ships through the sea.