The milk sign is a lot like milking a cow (or goat), but without the vertical motion - you are just squeezing the udder. You take your dominant hand, make it into a fist, relax, and repeat.
You will notice most babies have trouble moving all fingers together at uniform speeds, but any kind of repeated squeezing and relaxing of the hand is likely milk. If you want to distinguish breast milk from bottle milk, sign milk near your breast.
Milk would be one of the first signs that I would teach my baby, because it corresponds to something they find highly desirable, and so they are highly motivated. Also, teaching the milk sign helps reduce frustration, when your baby wants to feed but cannot communicate it effectively in any other way than being cranky. Make the milk sign before you feed your baby.
Once you have taught your baby a sign using real-life examples, it can help if you teach them the flash card that has the same sign. That will help your baby generalize the sign and understand that it applies to more than just a single instance.