You make a hole through your 'conker' (horse chestnut, not the edible type) and thread a string or a bootlace through it.
Then you take turns.
One holds their string still and lets the conker hang down, the other gets a swing at it with their conker. Whoever's conker lasts the longest is the winner.
There were all sorts of rumours about baking them, or soaking in vinegar or what have you to harden them up, but effectively it's the sort of game that a bunch of kids can play under a horse chestnut tree with relatively few props.
Using a steel 'ringer' in that circumstance would be the worst sort of unsportsmanlike behaviour.