WARNING This will take a little bit to get the hang of it
1) Start up the game

2) Host a Lobby for the number of people you want to play with

3) Select the course that you want to play
4) Select Dunk mode by clicking past classic and custom

5) Start match
6) Draw back as if you were taking a regular shot and release
7) As the ball is traveling towards the basket left click again for the ball to jump. THIS IS GOING TO TAKE A HOT SECOND TO GET THE HANG OF!!! So just hang in there for the achievements if you truly can't get the hang of it. If done at the correct time, this can dramatically reduce the amount of shots that you would have to take on a regular playthrough

Overall this mode can be very challenging to get a hang of, but once you do its quite a fun change up from the classic mode!

Have fun and keep putting!