The above solution explains what to do great. But it should be noted that this achievement is buggy. Sometimes, it simply will not pop even if you follow the instructions exactly.
I heard some theorize that this was based on being host or not, but I found it to simply be random. I tried for it twice in one game where I wasn't host - first time it didn't unlock, the second time it unlocked fine and I did nothing different.
If it doesn't unlock for you at first, try a few mores times and see if it does.