It's not that simple. Most of the time, the seceding nation and the nation that it ceded from are usually left alone to figure out how they're going to go about doing this. If the ceded nation protests, and does not allow independence, there is little the UN can legally do to stop it. Even then, the UNGC must also give a majority vote in order for a successful independent nation. A seceding nation can force the issue through the UN, but if it loses, then that's the end of their ambitions, which is why most separatists tend to not take the UN seriously.
I don't want to get into details, so I won't. I suggest reading up on the subject first, before opening such threads. The title is an over simplification of what the actual rules say.
