Android has private settings to prevent that (or at least cynagonmod has it). Well its not hard on GS2. Only depends what OS you use.
https://wiki.cyanogenmod.org/w/Install_CM_for_i9100
Only issue i sometimes have is finding the right Recovery. You have to start with that. That is these days only thing you need to flash. Then you can just copy past CM and install it from the recovery. If CM13 doesnt work start with CM11 and update it to 13.
if you are busy anyway try this too: