it's hard to explain how to roll a joint unless you actually demonstrate it, and it's even harder to figure out how to roll a joint... but once you finally grasp your mind around the concept, you'll roll well every time. it's like learning/riding a bike save the training wheels: difficult at first but once you get it you're set.

all i can say is ask someone you know that can roll to show you how, and have him watch you as you practice so you have someone to tell you what you're doing wrong, what you're doing right, etc. and good luck, practice makes perfect!