This is a very easy trick to learn. First you will need to be comfortable with your ollies. Now apprach what you are attempting to slide. Approach from the  side ollie up and when you get over the object turn your hips to where you are over the object and land on it with one truck to the left and one to the right. Now ride out the slide all the way to the end of the object and when you fall off it rotate your hips turn and land!