Where do I catch the MTR at the Hong Kong Airport to go to the Mong Kok station?

According to your requirements, you can take airport express to get to Tsing Yi Station first. Then transfer to Tung Chung Line to Lai King Station and Tsuen Wan Line to Mong Kok Station, Exit C1. The whole way takes around 50 mins. And it costs 60 HKD per person.