So once you have a low obstacle, ride up along side it with the obstacle behind your heels, then lift your front trucks over the obstacle and balance on it. This kind of slide is called a "slappy" slide, when you don't ollie up into it.
Just ride up along side the object with a fair amount of speed, pivot onto it, slide, and then pivot back off and ride away. You don't have to pivot back off of it if you don't want to - if it works, you can just slide to the end and then turn as you get off of the object.
Try slappy slides like these for a while, and get the hang of balancing as you slide. If you fall, do NOT catch yourself with your arms. That's a bad habit in skateboarding - it's really easy to break your wrists or arms that way. Instead, try to catch yourself with your feet and run. If you fall backwards, that's a little harder to do. In that case, try to land on your shoulder or backside. Landing on the rail that way can hurt - but at this stage, it shouldn't be too much pain. Get up, shake it off and make sure you try again.
