How did Stalin control the Soviet Union?

Finishing with the Russia´s History we had to do oral presentations. With my group (Felipe Draghi and Facundo Ramirez) we had to talk about, How did Stalin control the Soviet Union.
Here you have our presentation, and the link of our document were we put all the information.

This is the link with the information:
https://docs.google.com/document/d/1tLADKOsR48gczW9ynBlboHtTxlIv1urdj8C33rWexQM/edit?usp=sharing