You have to have Moon Gravity.
Go to on East L.A and go to where you had to Hang on a Helicopter as a Mission... If you haven't Yet, Go to where the Bus is and and go across the street and go to that Door that can take you up that building.
From there if you go straight you can go up another Sorta like Room, you'll see stairs that will take you up.
Look to your right you'll see a building a little higher than where you at.
Go Back A Little to gain speed with your Sk8 board and then Bone Ollie and when you touch the Building Wall Run Up to it and then Front Tuck on top of it, it may take a Few tries.
Once up Go straight and at the edge Front Tuck and You'll end Up on a Bridge it goes Pretty Far Both ways so it's Cool..