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.
Again and more are both often simplified to thumping hands together , 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 it' 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 your baby how to sign again for experiences, like being pushed on a swing or being tickled. Teach the related concept, more, when asking if your baby wants more food or drink.