planning to travel to Sanya late dec. 2023

Hi, we are planning to visit Hainan ...we are Belgium passport holders. Do we need a travel agent to get a VOA if we start from Hong Kong? if Yes, do we first have to visit a travel agent in Hong Kong or how does these things works? How is the weather late December? Which is the easiest way to get from Hong Kong to Sanya?
We are elderly people but still like to travel independent...we don't need a visa to travel to HK ...