STEP 1: Approach the obstacle either parallel with it or slightly at it. Do so with comfortable speed.
STEP 2: Ollie up in the air and get over the top of the obstacle. Come down and land with the hanger (or axle) of your trucks on the edge of the obstacle.
STEP 3: Keep your balance directly over your board, and slide as far as possible.
STEP 4: When you are ready to come off the obstacle, let your nose up a little bit and turn away from the obstacle. You should then be able to drop off the obstacle and ride away.