Throughout your travels, you may encounter a throng of excitable townsfolk baying for one of two pummeling pugilists. These are (unsanctioned) fistfights, and there are 13 to attempt (four in Velen, four in Novigrad, and five in Skellige), each with varying degrees of difficulty. All are optional, but you must defeat the first three opponents in each land mass before the fourth can be faced. Should you defeat all 12, a thirteenth and final bout is offered.
In most cases, simply find the bookie (or fightmaster in some parts of Skellige) who oversees the fight, and speak to him. He’ll ask how much you want to bet on yourself. Choose an amount, before you’re introduced to your rival. Then, better him in a one-on-one fistfight. No weapons are allowed.