Here's how to do it: Approach the obstacle at a small angle (either frontside or backside) with a moderate amount of speed. Ollie onto the obstacle as you would for a regular 50-50 grind, except you have to ollie a little bit higher than the obstacle to get into the actual grind position. Now, your back truck should slam down onto the edge of the obstacle and start to grind, this is were balance is very important. Keep most of your weight on your back foot in order to keep the grind position, keep on going until you feel that you're losing speed, pop a mini ollie out or just ride off the end of the obstacle (if there is one). After mastering this trick you can improve them by making them crooked, or performing them down obstacles that are on an angle (like handrails) so your board is parallel with the ground but still in the postion of a 5-0 grind.
This trick is fairly simple if you have mastered regular 50-50 grinds, after you perfect the 5-0 grind, it's time to move onto the nosegrind or crooked grind.