use ginger (green spout one); vinegar (I use those from "pa chin' there is a retail outlet near the Mongkok market.
Wash the ginger and peel off all skin as usual, cut into slice
use a wok, heat up the wok (dont put any oil) and put the ginger slice into it, stir and wait until the nearly all excess water is dried up and the ginger is cooked (usually about 10-15 min)
Transfer the ginger to a plate and leave it cool.
Heat up the vinegar in a separate container and put it to boil, add appropriate amount of sugar to taste and a pinch of salt. (u have to taste the vinegar sauce to suit your taste)
Use a wide month container, put all ginger in and pour the vinegar which virtually cover the ginger. Store overnight and the preserved ginger is ready to serve.
Usually, I will make a lot of them and store in the freige and it last at least 6 months without turning bad as you have added salt.