For starters, try with the same object you used with the last step. Ride up along side the object, with the object behind your heels. Ollie and turn 90 degrees facing forwards, landing with the rail or whatever you are sliding on in the center of your skateboard. It's a good idea to have your feet above your trucks, so that you can balance easier. Keep your knees bent, and make sure you stay over your board. Don't lean back or forward - just keep your shoulders over the skateboard.
When you come to the end of the object (rail, curb or whatever), turn your shoulders the 90 degrees back the way you came, land and ride away. If you want to get off the obstacle early, then push down on the tail of the board and lift the nose up and over the obstacle, land and ride away.
This might all take some practice, but that's how skateboarding works! Once you feel like moving up to higher rails and curbs, then go for it! But make sure you take the progression slowly - don't try something too much higher than anything you have already tried. Go slowly. You'll need to be able to comfortably ollie higher than the actual object you are trying to slide on - that's a good way to judge.
