Make your way by bus to Tapachula. From there, take the once every half hour bus to Cacahatan. From there, another once every half hour bus heads the 23 kms to Union Juarez. This is as far as public transportation goes.
From the main plaza in Union Juarez, walk (if you're lucky, you may get a lift) north along the paved cobblestone road to the villiage of Cordova, and then continue to Taliquan. From Taliquan, find the stone monument that marks the trail. From the stone monument, the well used trail heads along either side of the Mexico/Guatamala border. The border is easy to identify as its marked with a highly visible clear cut line through the forest, and refered to as La Linea (The Line). This is an interesting walk, and it passes through several tiny villiages and farms that can only be reached on foot. There are several places to get water at the farms, and then higher on the mountain at some springs at 2800 meters (off the main trail aways. Eventually La Linea reaches a cliff at the beginning of a pine forest (about 3350 meters or 11,000 feet). From here, the easiest route is to head north through the pine forest (there are several minor confusing paths here) until you reach a very good path to the summit of Tacana. On the summit, there is a spectacular campsite in the crater.