Hello. I'm so sorry for your suffering. I understand your decision to ctb. Hopefully you're not rushing into it and have exhausted your options.
It is risky if you fail. Brain damage is a very likely outcome. If you're found early especially, as this is the biggest risk. You want to be alone for at least an hour to be safe. If you're found, say 6 minutes into hanging, you could be quadriplegic(completely paralyzed). If this happened, you could only hope that your parents or power of attorney would do you mercy. I don't want you to be worse off than you are now.
Like one4all said, practicing can be dangerous. This is how people die in the choking game. It's easy to just slip into unconsciousness before you even know what's happening. So only practice if you're positive you want to die and are truly ready .
Remember, there's no shame in backing out. There's always another day to try. But if you're truly ready, I completely understand.
I'm here if you need to talk at all. If you should attempt though, I truly hope that everything goes well for you and that you find peace.