First, remember that a separation agreement does not end a marriage, only a divorce can do that. However, an agreement can still be a very important and powerful document. It can be used along with a divorce to set out all the details regarding the split. In that case the divorce ends the marriage but all the details are dealt with in the agreement.
It remains powerful by itself and a divorce can follow in time or not at all. In some cases the separation agreement is enough and couples will get back together or stay married but also separated.
Filing the agreement is sometimes done with a divorce but usually done when one party needs help enforcing it. It does not need to be filed with the court to be legal as long as it is correctly signed.
In some provinces certain terms of the agreement are not enforceable unless each spouse receives independent legal advice (from a lawyer) prior to signing it or at least signs a waiver of independent legal advice which is witnessed by a lawyer. It is always a wise move to spend a little extra to have a lawyer go over your agreement with you before you sign it.
Your spouse can ask the court to overturn the agreement because they did not know what they were signing if they did not have independent legal advice. So it is a good idea to make sure they get independent legal advice too.
The most important things to cover in your separation agreement are the details regarding the care and financial support of children, the issue of spousal support, and the division of your property and debt. Obviously if there are no children, low income and little property, this is easier than if all of these matters must be dealt with.
The most important things to remember are: 1. to understand what a separation agreement can and cannot do; 2. to make sure that it is signed correctly after receiving independent legal advice; and 3. to make sure it deals with the important issues of children, income and property.