It was difficult in the beginning. He wouldn't stay at the corner and would always escape! =P So I had to keep bringing him back to the corner. After about two months, he seems to understand and would stay in his corner. Just look at him, ended up lying down cause he was quite tired here. We had just got home from school!
Last weekend, I also started establishing the naughty corners outside the home. It wasn't intentional though. He was being very difficult while we were having dinner at Takashimaya. I warned him that he would have to stand in the naughty corner when we got home but that did not deter him. So I just walked him to a quiet corner and made him stand there. He was crying and kept turning his head to look at mummy who was walking away from him. After that, he was an obedient boy once again!