1. Get Data.
2. Do lots of research
3. Find edges
4. Bet and win

I find sport very hard to make a profit from. I have tried lots of angles but its very hard to find something not being exploited already.

Also for NBA and NFL there is a lot of research being done by researchers at universities. These are all published and widely available and a lot of people jump on these.

Saying that there is heaps of data so you can do very indepth research for example the NBA has play by play data available for free.