Sit in a circle with either a bottle (or two or three) and some shot glasses in the middle or each person has their own drink.
Each person picks a 'war sign' which can basically be any signal or sound (decide which before you start). E.g.. wiggling your hands in the air or calling bee-baa bee-baa etc. Each round you pick new signs.
Run through them just once and then start with one person who does their own signal and then does someone else's who has to carry on by doing theirs, then another person's.
When someone screws up like they don't realize it's their turn or uses the wrong signal (happens the second/third time around) then they drink.
It's really great if you add general drinking game rules like thumb-master, pointing, spillage, slidage etc.