Best way to build up calluses on your knuckles?

My mom always told me that it’s good to punch things since it builds up your calluses on your knuckles making them hard and when you punch to cause more damage. She’s a 2nd degree black belt in two styles but she’s looking for a school for wing-chun (a Chinese fighting style which I wanna learn) and that’s something she always told me, but I want to know the best way to build that up…So any suggestions?