The anime never goes deep into it, though specifics were given in other material. If you want to know the conditions/rules/vow Chrollo has set for his nen stealing, I'll list them below, otherwise can just ignore this comment.
1. He must witness the Nen ability in action with his own eyes.
2. He must ask about the ability and be answered by the victim.
3. His victims palm must touch the handprint on the cover of Bandit's Secret.
4. All of the above must be completed within an hour.
All of these conditions where met when he sat with Neon in the resturant/cafe booth, she even wrote his fortune inside Bandit's Secret when he passed it to her, so she touched the cover with her hand. All of it happened in well under an hour.