First I'd recommend making the prison population about 75, incase some refuse to riot. Then, set it to lunch time so that everyone moves to the canteens, next lock the doors and wipe the regime so that nothing gets done. This will make your prisoners starve, demand freedom and will eventually result in the prisoners kicking off. Since they're all locked up together, they will incite eachother to riot, and once it reaches 50+, then flush the area with riot guards and armed police (depending on your research).
If you don't have a prison with 50+ prisoners, you can edit the speed of time in the config file of your save, so speed up time so that you can order prisoners super fast (don't worry about supplying them with rooms etc, you want them to kick off afterall).