First of all, you can choose any skater that has a special trick that would go slowly with the "Cool Specials" cheat on(EX: Mctwist, Quad Heelflip). Make sure you go on "Too Easy Mode".
Then you can choose any level to skate. When the level starts, turn on the "Cool Specials" cheat. Then get special and go to a rail.
Jump and quickly do your special trick, and while he/ she is doing their trick grind the rail.
You should end up grinding the rail in slow motion. Now you can do anything in slow motion except a trick that goes slowly with the cheat on.
If you fall, go out of bounds, or do a slow motion special trick, you will come out of it immediately.
Don't worry, the game can't crash and the graphics will be a little bad when you're moving, but that's normal.