To sign again, flatten out your leading hand. Bend your second hand at a right angle, then tap it against the middle of the palm of your first hand.
Dads will find this similar to a timeout signal, but note there are two important differences. First, the moving hand is bent at a right angle, not straight like in a time-out. Second, making the again sign will not allow a three-minute cessation of all activity, wherein you may gather additional alcoholic beverages and salt-laden nibblets.
Again and more, are both often simplified to thumping together hands, and they often blur into one, given their similar meaning.
Again is so similar to more in meaning, that again is not often taught until a baby is more advanced. I will often not teach 'more' and just use 'more' for the same purpose. If I teach again, then I will tend to make it about experiences - and I will let 'more' be about food. So sign again after the first wave of a tickle attack, before the second wave strikes. "Want to be tickled again?" "Mommy is tickling baby again."
Teach baby how to sign again for experiences, like being pushed on a swing or being tickled. Teach the related concept,more, when asking if baby wants more food or drink.