you wouldn't want to go there without a local; Yuen Long is in the NT.
I remembered taking a bus from Mongkok to somewhere; and from there, take the Kowloon West rail??? to Yuen Long;
not sure whether you can take a taxi from TST to Yuen Long; remembered that I paid something like 250 HK$ @ 1 am from Yuen Long to TST.
better stay along Nathan Road
if you still insist on visiting YL, will get the directions from my godson's father; they are staying in Yuen Long