To unlock the Glam Dunk trophy in Trials Rising, you will need to unlock the Ever, Everer, Everest track, and then reverse at the very start until you see the basketball floating in the air. Reverse so that you are behind it and then press and hold the Triangle button to bailout towards it.
Once you have the basketball, keep holding the Triangle button and you will be lifted to the top of the mountain. At the top, you will see the basketball hoop. When you are above it, let go of Triangle to drop the ball, and then immediately grab hold of it again and try to get it into the hoop.
You can see a video below (credit to Gordinio on YouTube):