By bilking people out of money and trading on his dad's name and connections. Trump didn't start at the bottom. Thinking he is successful on his own would be like congratulating somebody scoring a game winning run from 3rd base on a walk off home run. Without the hundreds of millions and the right connections from his dad, among other things, he wouldn't have gotten anywhere.
Kind of telling that he would be worth more if he had taken the money his dad gave him and put it into indexed funds and done nothing else. Instead he embarked on decades long projects and developments that have yielded surprisingly little. But because he had his dad to bail him out behind the scenes, it perpetuated the con job he was successful. That's when he really believed it and people lent him money and he started failing for real.
The reason he isn't flat broke is because of advice he gave convicted fraudster Bruce McNall. McNall said that Trump told him his mistake was he didn't borrow enough. If you have enough people invested into you from the big banks or whoever, they can't let you fail. That is how Trump has basically gotten away with being who he is. It is in some people's interest that he not fail. So in the meantime he is just a clown living on credit until people decide he isn't useful anymore.