As part of a longer trip to Indonesia, I wanted to visit Tana Toraja but found that sorting the logistics of getting there was miserable. I did not take a tour (not my thing), but wanted to get to Rantepao and hire a guide on arrival. Things that I wanted to know before I left:
1. Bus schedules: Like others have posted, buses travel from Makassar predominantly around 9am and 9pm. They cannot be found online, and even calling was difficult. I wanted to take a day bus rather than overnight, and went with the Litha Bus (which was great) largely because it was the only one my hotel could reach to find a schedule. Apparently Bintang Prima and others run the route, though I was told only in the evening (that could be wrong). Litha runs at 9:30, 12:00 and 13:00. It is at least a solid 10 hours; I took the 9:30 bus and arrived in Rantepao at 20:00. If you are planning to travel in the morning, you should be around Makassar the night before (planning to take a noon or 13:00 bus would get you Rantepao at midnight)
2. Location of the bus: This seemed a lot harder than it needed to be (not easily google-able, for one thing). The buses to Rantepao from Makassar do not leave from a central bus station in the city. Each company has a separate station along Jl. Urip Sumoharjo, which is about a 25 minute ride from the city toward the airport. Alternatively, the buses also stop at "Daya Bus Station" which is about 20 minutes further, between the originating station and the airport. If you stay near the airport, this may be reasonable, but if you are in Makassar, you will have better luck with each individual station. Either way, plan a fair amount of extra time prior to departure.
I hope these are helpful to those traveling to Toraja, it is worth it.