There is lots of information how to do this on flyertalk.
If you take the train you should be in Kowloon around 60-70 min after arrival of your flight. You should be back at the train station not later than around 90 min (very close - 120 min to have some buffer) before your flight - 10 min wait, 25 min on the train, 20 min to go thru security and immigration, 10 min to the gate, need to be at the gate 20 min before departure