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.
More is best taught during meal times. Give your baby small portions, then make the more sign before you give them another portion.