Hi BlueValentine,
I live in Sheung Shui and I can tell you all I know about commuting from HK island to Sheung Shui in the early mornings.
FROM WANCHAI
There is a red minibus from Wanchai to Sheung Shui. I can't remember exactly where it is but I believe its on Hennessey Road and its right outside of the Hang Seng on the side heading towards Causeway Bay.
FROM CAUSEWAY BAY
There is indeed a red minibus running from Canal St Bridge as mentioned in an earlier post. Its at the end of the street near the fire station. I remember its $25.
However if you are near Wanchai it may be easier to walk to the Wanchai minibus. Just walk along Henessey Road towards Wanchai and you will evtually see it.
FROM CENTRAL
There is no firect minibus back to Sheung Shui form Central. Your options are to get to Wanchai or to Mongkok for interchange. The red mini bus to Mongkok is at the bottom of D'agiular Street, around the corner on the left of Queens Road. It is pretty much outside of the Body Shop. I think its $15.
FROM MONGKOK
There are two minibuses from Mongkok to Sheung Shui. The first from Mongkok Road. Again quite easy to find. It is around the corner from Sai Yeung Choi Street on Mongkok Road. Usually quite a long quite at nights but its the fastest route to Sheung Shui. Usually around 25 - 40 mins depending on speeds.
The other is on Argyle Street. Again pretty easy to find. Just walk up Argyle Street in direction away from Nathan Road and you will see it. It is on the side going towards Nathan Road. Also this bus goes to Tai Po too, so takes longer time on the road, but usually shorter queues.
Hope this is enough to get you going. If you have any questions just ask. I also know my way pretty well commuting to Yuen Long, Tuen Mun or Tai Po if you want to know just ask.