You can start a kill contract by talking to Weasel in the underground tavern to the South-East of Starry. As of now, we only have easy bosses, but don't let that fool you, they are not easy. Purchase a contract from Weasel and then go talk to the party creator. From there, you can create a party, ready up, invite others, and start!
Once you start, there is no going back. If you die in the arena, you will lose your items. Contracts are not refunded and are used as soon as you go into the arena. Any death by disconnect, server restart, or any other event are not going to be refunded.
Do be warned, bosses have various mechanics that differ from each other. Learn from your deaths and perfect the strategy to beat them!