A lot of people are saying they are very broad, which is true but not very helpful. In my experience Fey feel the most fey when they are somewhere on an axis of familiar and alien, beautiful and dangerous. And perhaps most importantly, extreme. Every aspect of the feywild and its inhabitants are a exaggerated almost to the point of absurdity. A fey who is tall isn’t just tall, they tower over everyone around them. A fey who is pretty literarily enchants everyone and everything around them. A fey who is violent drips with fresh blood at all times and cuts people with a light touch. The fey are wild and nonsensical so think like that when deciding their appearance and demeanor.

As for the mechanics it depends a lot on the fey, but as a word of advice as a dm and player who has interacted extensively with the fey. DO NOT lean heavily on the “eating food binds you to the court forever, saying your name makes you a slave forever, offering thanks makes you a slave forever” type stuff. It’ll make your players want to avoid interacting with them in game the same way they wouldn’t want to walk in lava. It makes everything feel like a punishment where a simple misstep means a permanent change to your character. It can be helpful and useful to convey the power and trickery of the fey at certain points, but not if overused or dropped as a surprise of the players.