I've only used double edged but I've read that single work better, they are less flimsy. For my attempt I used #10 scalpels, they are quite handy. Just ordered them online. One of the issues with cutting is blood coagulation, to combat that you can take aspirin for a week beforehand (I took 3 a day), it thins out the blood. Be careful not to take too much or it can cause internal bleeding. Also during the attempt I drank some alcohol, it's supposed to help with blood flow, and I aimed the shower head at my wrist. What you want to do is hit an artery, the closest one is radial near your wrist, where you can feel the pulse. Better to go lengthwise along the long bones in your arm but either way would work. The difficult part is cutting in the right spot and deep enough. This method has quite low lethality rate due to this, so if you want better chances of success I'd recommend looking into other methods