Before learning how to slide, you will need to find a nice piece of flat land to practice your sliding stance. Practicing your sliding stance on flat land makes things easier, plus you don't have to worry about the whole falling off while going fast thing. If you're finding that the flat area isn't enough, move to the grass. It's even easier to balance on a good patch of grass.
There are many different ways to slide. The easiest method is to just think about how you have seen other people do it, and practice the movement. This consists of squatting down, grabbing your board, and shoving it out from underneath you using your back foot. This is easier said than done. Practice the motions a few times and when you are feeling good about them, try it on a slope.
The first time you try sliding, you will probably fall. This is unavoidable and is a part of the learning process. The other bad thing about learning to slide is that you can't really do a good slide unless you are really moving fast. This makes sliding terrifying.
Many longboarders prefer to use loaded longboards when riding downhill simply because they are one of the only boards that were designed for high speed skaters. The decks are shaped specially to give the user more control than they would have on a regular longboard. The positioning of the trucks is different as well. It's all about giving you easier ways to get the board sliding.
Now you are pretty much ready to start sliding a longboard. Make sure to practice your motions really well, or when you get to the hill, you might get some unexpected results (death, serious injury, or maybe just some bruises and scrapes.) You can do it, though. It is easy. Just wear your safety gear and a helmet when you try it the first time.
