First you need to go to the top of the hill where the gym is at. Skate over towards the pic nic table at the start. Then keep going up and towards the hill that takes you down to the pools area. You should eventually see a rail coming from the grass that goes over to a little roof. If you grind over to it, and jump onto it while on your board, you will float off of it.
Now try to get on the roof while on your board. Once you've done that, jump up into the air and get off your board and then hold DOWN and a GRAB. You sometimes will land into a LFG and eventually will fly off of the roof.
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