How do I train my puppy to pee on the pad?

My friend gave me her 5 month old male poodle mix. He seems to be already used to peeing outside on grass, how do I teach him to pee on the pad and not outside? I know what most of you guys might say, "Oh, take him outside because he is used to peeing on grass already." But my friend lived in a house with a large back yard where he could pee, but I live on the 6th floor of an urban apartment building.