/diy/ - Do It Yourself

Does anyone have any advice on how to use one of these guys to cut glass? I'm trying to cut some and I am struggling to get a consistent mark in the glass to break it on. In addition, whenever I break the glass, I do not get a clean, straight break. Instead I get a break that is uneven.
Do these cutters get worn out? I just used one that I found in my shed.