Josh can be found at the motel again ("J" on the map with a small question mark), so do the bouncy bouncy and then he'll tell you what he wants done. It seems the house you met him in front of initially, which is his, is now in foreclosure. He wants you to stage an accident there so he can collect the insurance.
Once the conversation is over, grab the jerry can from his car (you might as well use it, since it's a nice car) and head to his house. When you get there, jump over the gate and make your way to the grill in the back. Equip the jerry can and pour a trail of gasoline, following the line given to you.
After you're finished pouring the gasoline, shoot the gas to start the fireworks. You now need to escape, which you can do without drawing the attention of the cops, although you will likely do just that. If the cops are onto you, lose them to finish the mission.
