If you are trying a 50-50, pop off both feet from the side about 1.5 feet away from the rail. Basically jump sideways onto the rail. Once you get onto the rail, you want to be overtop of the rail. At first you will tend to either not hop on enough and fall off the side you jumped on or you will lean to far forward and fall off the other side. Trying 50-50s on a down rail doesn't have much risk as you will just fall off the side and ride away if you mess up.
For backside boardslides, you need to get the right amount of speed, not too fast or slow. This time it is more of an ollie onto the rail then popping off both feet. You want to approach the rail flat footed and ollie onto the rail by getting your nose over the rail and landing on the rail in between your bindings. Most people who first try boardslides do not get the rail in between their bindings.. instead they just slide on their nose and end up falling off the rail.
Its all about commitment, once you try it a couple of times its not as scary. Keep trying and eventually you will get the hang of it.