Train T806 leaves 16:13 from Foshan to Hung Hom Station Kowloon, arrives about 18:49, may I know where to take the train?

I just checked that the train leaves FS at 16:35 and arrived at Hung Hom around 19:31. You can go to FS Railway Station to catch the train and you can also buy the train ticket at the train station.

However, as it only operates every a day, it may not be the best choice. I suggest you take a bus or train to Guangzhou first. Then at Guangzhou East Train Station, there are many trains to Hung Hom directly. And I know the train tickets between Guangzhou East and Hung Hom can be booked online though Travel China Guide. Here is their email address:
bookingtravelchinaguide.com