To do the sign for more, flatten out your hands then bring your thumbs and fingers together, to make flat 'O' hands in ASL. Then, bring your hands together and pull them apart repeatedly.
Your baby will often simplify more, gleefully clapping their fists together. As they get older and more proficient, you may want to encourage the more correct form of flattening out their hands and creating the flat 'O' shape, as a fun way to help them develop fine motor skills.
More is one of the most popular signs with babies because it gives them a lot more control over their life. We start using more when eating or drinking to communicate when they have not quite had enough of something. So when I am giving our baby her bottle and she stops half way, I will ask her, "Do you want more?" while at the same time doing the more sign. Then give her back the bottle. Once your baby gets more advanced, we start using more in other contexts. For example, when we are playing a game or tickling our baby, we might stop and ask, "Do you want more tickles?" That kind of question-response pair is a great way to start getting your baby starting to learn how questions and responses work. Remember to do an exaggerated rising inflection in your voice when you ask the question, "Do you want more tickles?" Your baby is learning as much from your tone and body language as from anything else!
More is best taught during meal times. Give your baby small portions, then make the more sign before you give them another portion.