Here's how to do it: Approach the obstacle, parallel with it (either frontside or backside) with a fair amount of speed with your back foot on the tail and your front foot on the nose. Remember that the faster you are going, the further you will slide. Ollie up a little bit higher than the obstacle, and land so that your nose is on it, if you've done everything correctly, you should now be sliding. To aid your sliding, keep your balance by maintaining most of your weight on your front foot. As soon as you feel that you've slid long enough, quickly pivot your board forward/backward, hit the ground evenly and ride away smoothly.
You'll get a better understanding of the noseslide if you watch an experienced skateboarder perform it.