Step 3: Play
The object of the game is to smash your opponent's conker, or, failing that, knock it off the string.
Decide who goes first any way you like - toss a coin, scissors-paper-rock, pistols at dawn - and then one player wraps the loose end of the string around one finger, and holds their conker out at arm's length.
They must hold it still, without swinging or swaying, and without flinching when their opponent takes a swing.
The other player then holds the string of their conker in one hand, briefly restrains the conker in the other, and swings as hard as he can to strike the hanging conker.
If the conker is struck, the conker-swinging player gets another go.
If the conker is missed, the players swap over - the hanging-conker-player becomes the conker-swinging-player, and vice versa.
This continues until one conker breaks or comes off the string.