I climbed Santo Tomas from Fuentes Georginas a couple of weeks ago unguided and would like to leave few warnings and recommendations.
First point is that it is definitely not a 4 hour climb to Santo Tomas, but more like 12 hours. Do not do it in one day, but bring a tent! This is not just our group, but a group of professional hikers who told that it generally takes 6 hours to climb up and 4 to climb down even if you are in good shape.
As seen in the map, there is only one point where you must turn when you come from Fuentes Georginas. After climbing up the bamboo forest you will come to T-crossing (and a small path that continues straight). To left it goes to some kind of Finca, straight doesn't really go anywhere, right goes where you want to go (you see a sign post so thats probably what you would choose anyway). Then you will simply go straight and there will be no problem. The best camping spot and view on top when coming from Fuentes Georginas side is actually past the hill and little bit down to the other side.
When coming down from the top towards Fuentes Georginas there are different paths. You need to all the time keep on the left side of the mountain! We somehow sidetracked to right side and ended up to path that led to the steep cliff where you can fal down. Then we couldn't even find the way we came, as we had begun to go through forest that had an 'imaginary' path, and started heading left to reach the other path and the base camp below the last climb. It took us two hours to walk along the cliff to reach the grass area.
Also after the crossing to volcano Zunil there is another path that goes down and even has an arrow, but it is not where you want to go as it is a lot more steep and muddy. In the bamboo forest there was also another point to get lost when coming down, but there we realized quite soon that it was not where we came from and turned back to take another turn.
My general recommendation is to turn back if the path is not obvious, as the real path was obvious all the time and recently opened with machetes. There are really steep cliffs which might even take your life, and a German girl was lost for 4 days after falling down when she got lost.