Just start small. Ideally you can find a small urban style down rail to first try them on. Then once you are comfortable, move up slowly.
If you don't have this option, its just gonna take more balls. Just remember your spin comes from your outside arm. Pull it back behind you and throw it across your body up towards your inside shoulder. You want to go up to your shoulder a little bit because this will help you spin leaning downhill a little bit and put you more forward to land on the down rail and not slide back on your ass.
Then remember to keep your tips from catching on the rail by leaving your knees straight. Don't bend them too much once you're in the air because that will dip them into hitting the rail.
Just man up and be willing to take some falls. Its a fun trick!