It is also very important to brace your hand against the extension or at least the head of the ratchet to prevent cocking the socket. That's what broke your plugs. You can't just wank on the ratchet handle. Imagine it was a T handle, and you only pulled on one of the arms... You have to replicate the twisting action of a T handle with both hands evenly, directly in line with the threads.
When replacing plugs, use a section of hose fit on the ceramic to start the thread. This way you don't have the torque to cross thread. You should be able to wind it nearly down the whole way. Pull the hose off. Then carefully slip the socket over the plug so not to contact the ceramic. Feel to see that you have dropped the socket down over the hex part of the plug, and carefully twist it tight.... not like a lug nut... like as much force as you'd exert on a pesty pickle jar, only the other direction.
If you're leaking coolant, it's more serious than a plug. Was the first page in this story an overheated engine?
Gar