As best I can understand it, Flew's argument is that we can't really imagine ourselves outside our own dead bodies. The logic has to do with identifying who is watching and who is in the coffin. Since we can't really imagine it nor really identify the watcher, this proves that there is no such thing as an incorporeal "me".
I find the whole argument confusing and unconvincing. This probably simply means that I don't really understand it. I'm inclined to go with Hobbes on this; when people talk about "souls" and "afterlife", they are not wrong, they are simply saying words that have no meaning.