Gelatin (Knox or store brand unflavored stuff at the grocery store) works well. I have used in the past a half packet (I think that's about a tablespoon) dissolved in 2 oz of water in the microwave. Microwave for short periods stirring after each heating until dissolved. I don't think you want to boil it. Can't remember the reason why right now. Pour it into your cider in the carboy with a gentle stir with something sanitzed. It will look like milk, but give it a day or so, and it will clear that cider up quite a bit. Dave is the forum expert on ciders. I'm sure he'll chime in and correct me if I have written anything in error here.