On practicalities, there is only a handful of operators who run the trip, mostly based out of San Ignacio. What I've arranged in the past is for people to fly from SP to the Municipal airstrip on the first flight of the day at 7am. They're met there by a car or minibus and driven up just past Belmopan on the San Ignacio road to the village of Teakettle, which is where they turn off the road and head south towards the mountains on a variety of dirt tracks. Maybe 1.5-2 hours total driving.
Eventually the tracks peter out and you take to foot through the bush, fording a couple of shallow rivers until you get to the pool at the exit to the underground river. Then you walk and occasionally swim up stream. There are some practical tips you will need, so post for those nearer the time.
As Tommy says, the experience is out of this world. I don't know anyone who's done it who hasn't been in raptures over it, and found it to be the high point of their entire Central American adventure.
If you go over the maps available on this board you'll find some which show just where it is.