‘Separation’ usually refers to breaking up when you have been living together as a couple in a marriage-like relationship.
You and your partner do not need to agree for separation to happen. At least one of you needs to make a decision to separate, act on that decision and tell the other person.
You might need to be able to show that you have separated and when you separated. The date of separation may also be relevant in property settlement proceedings. You do not need to sign anything but it is a good idea to keep a clear record of what property you have at the time of separation, what you took with you and what you left behind.
You should always get advice. You can do so before or after you separate so that you can be aware of any time limits or other things that might affect you.
It is always a good idea to talk to a lawyer before you separate just to make sure your best interests are looked after, particularly if you have been in a long-term relationship.