broken toilet flange?

My plastic toilet flange broke completely. i need to repair it my self and i have concrete floors. the soil pipe is still intack but i still have the peice of exsisting plastic flange on the inside of the pipe. the guy at home depot told me to just secure this super ring onto the concrete and i'm good to go but it seems to easy and i'm afraid of cracking the concrete foundation and leaks. please help ASAP i need some peace of mind and ideas