Does anyone know what the methods are for preventing rebellions? The tutorial does not seem to give any clues about it

Keep up food supply for the city, put in some units and also an option is to build "control" buildings (read the tool tips for that). Food is essential though. If you garrison a city you can hover over the city morale to get info on current morale, morale trend and modifiers. Without food any city will riot though.

If you want to prevent riots, you need to have a small garrison, nightwatch(i think) as a building, and the most efficient way is to recruit Hostages. Then move the hostages to another city and you can prevent riots, on the city that the hostages came from. When i conquer a city i always recruit a combat unit and a hostage unit as well. No riots at all.

While nightwatches are not working properly at the moment and otherwise cost a lot of wood, i prefer recruiting a combat unit. Having combat units in every town also has the advantage of being much more flexible to react on raids.

Previously combat units were the best way to do it since money was so plentiful. In the current build I am still in favour of combat units since they are very useful for a range of things but the other options have become more useful. I don't yet have a favourite way yet and still haven't figured out the most efficient way.

To be honest, more cities tend to be more of a burden as time goes on. The recruits are meaningless for the most part since they tend to be the wrong culture for roman units (which I prefer) and the effort in maintaining order costs a large proportion of your resources. I'd suggest hulk-smashing your way through to the objectives most of the time using the bare minimum number of captured towns to run your supplies through.

I don't yet have a favourite way yet and still haven't figured out the most efficient way

Use hostages, its the most efficient way ;)

Yes, build hostages and send them to another city or fort.