Obtain a Maverick helicopter from the South Los Santos hospital helipad to perform these aerial challenges. Maxing out the "Flying" attribute on a character is recommended so complete the Flight School at Los Santos Airport before attempting the harder challenges. To complete a challenge you cannot touch the ground or any part of the bridge at all while flying under it. Completing all 50 challenges will unlock the "Close Shave" achievement/trophy.
View the following video to see all 50 bridge locations: