Skill analogy:
Nobody is born knowing how to play the piano, but we are born with the capacity to know how to play the piano. Likewise, virtues
You don't learn to play the piano by simply reading books and just studying the theory, you have to actually do it. Likewise, virtues
When you first start learning to play the piano, you follow the rules and try not to press the wrong keys - but you don't really understand what you're doing. In the case of virtue, we start by teaching children rules for behaviour and they just follow these rules because they're told to - not because they understand why.
But as you progress with playing the piano, you become able to play automatically without thinking and, eventually, you might become so comfortable playing the piano that you're able to improvise and understand what sounds good, what doesn't, and why. Likewise, by following rules for acting virtuously, it eventually becomes part of our character. Further, we begin to understand what virtue is and this enables us to improvise according to what the situation demands.