Easy. One person starts with saying the number,"one." then onto the next pereson saying,"two.", then on to the next etc. The object is to not say a number that contains a seven (say "57") or is a multiple of seven (say "49") if you do, you gulp. If it's a multiple, you chug if it contains. Keep going as it gets harder as the numbers increase. If it gets too hard, you can start over.
What adds to the fun is if you start over and some one is a little tipsy and says, "77." (has happened!) This person must do a shot as they called a number that is both a multiple and contains the number seven. (not to mention being a complete dumb-shit) Enjoy!