Start off with a small ledge or box rail ( you can use a regular rail but I find boxes easier) make sure it is waxed. start by standing next to it and jumping on to it on only your positive soul plate(the one away from your body). This will help you get your balance and the getting onto the rail bit better. Then get at a moderate distance away from the rail at about a 30 degree angle and aproach it with enough speed to be able to slide along it.

If you over wax the rail you will possibly fall forwards/backwards and hurt yourself(this will put you off) and if you under wax it you will not get the distance on the grind(if any) and It may end up as a face plant or a makio stall.