The staircase leading to the large Guan Yin image was still under construction when I visited the place (December 2017) but small buses took visitors there from the car park. Otherwise, you can easily reach it by wlaking on the road that goes behind the statue. Note that visitors can climb inside the statue by an elevator and reach in its forehead. There, the interior has pretty white stucco carvings and you can take a look at the view through an opening. The most interesting view is, I think, the one you have from the left eye of the statue.
To go visit Wat Huay Pla Kung (Big Buddha), you can also book one of the tours above.
Wat Huay Pla Kung (Big Buddha) is a temple and when you visit a temple in Thailand there are some rules to know and follow :