Typical leaf liners come in three different shapes. A regular beveler with lines, a triangle with lines, a pear or shield shape with lines and a semi rectangle with sharp edges. The beveler ones that I own came in a pair. The right and the left angle. The triangle, rectangle and pear or shield shape are just one tool that can be used on both sides. The way that I was taught is that you want to mimic the veins that coming from the stem outwards towards the end of the leaf. The natural process of growth would be from the bottom of the stem where the leaf starts it life upwards toward the tip of the leaf that represent nerves, if you will. The bevelers are easy, but sometimes confusing for some people who are starting out. You determine which tool has the 'right' angle you want and you slip the heel and bevel upwards. Switch bevelers and do the same to the opposite side. The other tools mentioned here are a bit more simple to use. You pick the flat side with the corresponding angle and bevel away upwards. Remember, you want to start out heavy in your pounding at the bottom and fade out as you reach the top. You might want to tilt the tool towards the center stem to avoid making a complete tool mark.
I hope I didn't confuse you. I guess I can walk over to my block and take some snap shots for you if I did confuse you?
Good luck.