You will tell the bus driver to go to kanchanaburi. About 2 hours.
But at Bkk. have a van to kanchanaburi. And I think that fast as the bus.
If you take a van, it parks at Anusavari BTS, you can ask some people or around there to kanchanaburi
When you go to kanchanaburi you will take a bus to saiyok.
Do you want me to take you from kanchanaburi when you came from BKk ?