We introdcued it at about 5ish months I think, just a Heinz Baby Basics training cup I think its vcalled, with handles and a non-spill thing, they have to suck to get anything out. It isn't too hard either, jazz got the hang of it pretty quickly, and it hasn't spilled once. The spout is fairly soft, but since she has teeth and chews on it we didn't want it too soft, and she hasn't bitten it yet.

We just fill it up with water and let her have it for the day. She used to just suck the water out and the dribble it everywhere (lol, that became a game...), but we still offered it at every solid feed, and throughout the day and now she drinks a whole cup throughout the day, we just leave it on the floor with her and she'll find it when she's thirsty, or we'll offer her some if she hasn't had a drink in a while.